March Browns are large, early-season mayflies with mottled brown wings and a brown-to-amber body. The dry rides high on the surface film, imitating the dun stage that trout key on during the cool weeks of spring. There is an Eastern March Brown and a Western March Brown, and both fill the slot before the smaller spring mayflies and caddis get going.
Tie or buy it in sizes 10-14, color brown. These are big bugs by mayfly standards, so the fly is easy to see and easy for trout to commit to. The hatch comes off best in clear to slightly stained water, low to normal flows, with water temps in the 48-60F range.
March Browns often hatch sparse and sporadic rather than in a thick cloud, so trout look up for them between other activity instead of locking onto a steady drift. The duns are slow to leave the water in cool air, which gives fish a long look. Fish it dead-drift through riffles, runs, and the foam lines along soft seams. A clean, drag-free float matters more than an exact match - when a March Brown rides by undragged, a trout holding in cool water will move for it.