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Mayflies can be found emerging every month of the year in the Yellowstone area. They are particularly abundant from May through October and are an important food source for trout during this time period. Mayflies are responsible for many of the great fly fishing opportunities that arise during the season, and a knowledge of their habits is invaluable for an angler.

Mayflies inhabit every trout stream and the number of species a stream can hold is often amazing. Our friend Dan Gustafson has found over sixty species on the Gallatin River alone, a sign of remarkable diversity in habitat. Of course not all rivers support so many species, and not all species are important to fly fishermen. Some mayflies are not abundant enough to warrant individual concern, while other species are so closely related in behavior that they can be considered synonymous by fishermen. Other mayflies are simply not available to trout in quantities large enough to require any attention from fishermen.

The species we discuss are responsible for virtually all of a Yellowstone angler's mayfly fishing. The list is actually quite short, and if you do not fish certain rivers or at certain times of the year, the list shrinks even more.

While each species has its own peculiarities, most demonstrate a preference for similar emergence and egglaying conditions. Knowing when the best mayfly activity will occur is as important as knowing the individual traits of a given species.

The heaviest mayfly emergences generally take place in overcast, cool weather. Misting rain or snow showers are ideal and stir the blood of all fly fishermen aware of this fact.

Though we can find no substantiation for this theory among professional entomologists, every experienced angler we know is in agreement on this point. Entomologists tell us that mayflies may actually prefer warm, dry conditions, and that they hatch just as abundantly on those kinds of days. We've been told cool weather simply concentrates the emergence over a short time period, and that during warm, dry days just as many mayflies emerge but do so by trickling off over longer periods.

We have trouble accepting that theory because it runs counter to years of observation by so many people. While inclement weather is not a strict requirement for good hatches (we have all seen great emergences on nice days), there is no doubt in our minds that many more flies come off when the weather is bad.

Of one thing we are sure: the best fishing during mayfly hatches is definitely on days when the weather is poor. If not because there are more mayflies, then certainly because they ride the water longer in cool weather, and suffer more emergence defects. Both these factors give the fish a better chance to feed on them. Too, trout in general (and browns specifically) seem to feel more comfortable feeding under overcast sides. This makes approaching and casting to them easier.

Of the mayfly spinners important to fishermen, all need moderately warm, calm conditions to lay their eggs. Wind stronger than a slight breeze, cold temperatures, or any precipitation precludes the spinners from reaching the water. Attention to the weather then, as well as learning the habits of the prevailing mayflies, can be very important to successfully fishing Yellowstone mayfly activity.  Mayflies are most vulnerable and available to trout while hatching and during egg laying and subsequent spinner falls.


Baetis (Blue-Winged Olives, or BWOs) emerge best from I I A.M. to 4 P.M. on overcast, rainy, or snowy days. The cooler the day, the later the hatch.

Rhithrogena spinners are sometimes important on calm, warm evenings during July and August.

Pale Morning Duns (PMDs) generally emerge at the most comfortable time of day: noon to I P.M. on snowy days; 9 to 11 A.M. on sunny, warm days. Spinner falls are best on cairn, warm mornings, 9 to 11 A.M., and ag?in during the evening, 7 to 10 P.M.

Green Drakes emerge from 10 A.M. to I P.M. Fall Green Drakes come off from I to 4 P.M. Spinner falls (seldom encountered) occur from 7 to 9 A.M.

Brown Drakes hatch from 71010 P.M.; spinner falls coincide with emergences.

Flavs emerge on clear days from 8 to 9 P.M. In cloudy, rainy conditions look for them from I P.M. on. Spinner falls occur from 7 to 9 p.m. 

Gray Drakes on Slough Creek emerge sporadically all day long, beginning as early as 8 A.M. Spinner activity is better coordinated and goes from 9 A.M. until noon. On the Yellowstone River, spinner falls occur twice each day, 8 to 10 A.M. and 7 to 9 P.M. 

Callibaetis duns emerge on lakes from 10 A.M. to 2 P.M.; spinner falls occur from 11 A.M. to 3 p.m. 

Tricorythodesshow on the Madison River between 10 A.M. and noon; spinners are more important than duns. 

Pink Ladies hatch from 4 to 7 P.M. 

Attenella margarita duns and spinners can appear together from 9 to 11 A.M. Spinners may fall on warm, calm evenings as well. 

Serratella tibialis duns come off between II A.M. and 3 P.M., with spinner falls from 7 to 9 P.M. 

Heptagenia duns, when found, emerge between I and 5 P.M. Spinners fall best on warm, calm evenings.

Caddis emerge best on warm, calm evenings. Egg-laying activity usually occurs at the same time. However, in their peak periods caddis may also be found laying eggs in the morning. Trout recognize both emerging and egg-laying periods and feed best during these two events. There are two important exceptions to this:

Lepidostoma on the Gibbon River often emerge during the early afternoon on cloudy days.

Hesperophylax caddis on the Yellowstone River usually emerge from 8 to 9 A.M.

Stoneflies, such as the Salmonfly and Golden Stonefly, have their strongest egg-laying periods on warm, windy, sunny days from 10 A.M. to 6 P.M. These clumsy fliers are most available to trout when they bounce on the water while depositing their eggs. 

Little Yellow Stones lay eggs in the late afternoon and early evening, 3 to 8 P.M. 

Damselflies prefer warm, sunny mornings and afternoons, 10 A.M. to 3 P.M. Look for migrating nymphs and newly hatched adults along the shoreline. 

Midges are important throughout the Park. Be prepared to fish midges at any time of day, on Midge any water.