Aravaipa Creek is one of the few, small, permanent desert streams in Arizona. It has diverse fish and invertebrate faunas but the invertebrates are not well-known. I collected benthic and 24 hr. drift samples there during 1980-1981 and found 31 taxa. The most abundant species were Baetis insignificans and Choroterpes inornata (Ephemeroptera), which exhibited the nocturnal drift periodicity characteristic of drifting invertebrates studied elsewhere. In the laboratory, observations of the drifting behaviors of five species of mayflies were made in 5 and 10 cm/s current speeds. These provided evidence that drifting insects are not necessarily swept away by the current (accidental drift), but are probably exhibiting a specific behavior triggered by local conditions such as lack of cover or food (active drift). I also tested the relationship between drift rates and the presence and absence of food (periphyton) using B. insignificans and C. inornata. With both species, the number of insects drifting increased significantly when periphyton was not present; therefore lack of food may be an important cause of drift. The sizes of the nymphs used in these experiments were determined; I found that drifters were either similar in size to non-drifters (C. inornata), or that drifters were larger than nymphs that did not drift (B. insignificans). These results suggest that direct competition by size does not play a role in determining which individuals drift. Drifting appears to be a dispersal mechanism for benthic invertebrates; it maintains the numbers and distribution of the benthos according to the rapidly changing distribution of their food resources which occurs in streams. This process fits some recent models of population regulation by dispersal.