This study examines the effects of substrate and predator type on microdistributions and drift responses of nymphs of the mayfly Heptagenia hebe in a warmwater sandy stream in northern Michigan. Nymphs were significantly more abundant on cobble/boulder substrates than on gravel/pebble or woody debris. Mayfly densities among these substrates were unrelated to densities of predaceous stoneflies (perlids) and hellgrammites (Nigronia serricornus). Nocturnal drift, measured instream from enclosed substrates, was significantly lower from cobble/boulder substrates (0.1%) than from gravel/pebble and woody debris. Drift rates were unrelated to stonefly, crayfish, or fish (mottled sculpin and hornyhead chubs) densities, but were positively correlated with hellgrammite densities. However, experiments in artificial stream channels revealed that propensity to drift was strongly linked to substrate type, not predator type (hellgrammites and/or stoneflies). Cobble/boulder substrates apparently function as sinks (where immigration > emigration) for dispersing H. hebe nymphs in sandy streams with limited suitable habitat.