The drift of stream insects and other invertebrates refers to their down stream transport in stream currents. While stream invertebrates are adapted by various means for maintaining their position in running waters, it is to be expected that the occasional individual will lose its altachmeIt or orien tation to its substrate and drift downstream. However, it IS onlywithin the last 20 years that observations have been made of large number§-of aquatic insects in the drift. The quantities have been large enough to bring into question the stream's productive capacity to withstand such a high rate of attrition and the possible necessity for an upstream return of some kind. Furthermore, it has interested many ecologists that the drift usually occurs in some type of diel periodicity. and that probably d�adian rhythms are involved. It should be emphasized at the beginning that a drif�--fauna, as distinct from the bottom fauna, does not exist and is, in fact, an inappropriate term. Drifting is merely a temporary event in the life of many members of the bottom fauna or other substrate-oriented populations. Among the earliest studies on invertebrate drift were those of Needham in 1928 (84), who was primarily concerned with the drift of terrestrial in sects that fell onto the stream surface, as a fish food source. But, in captur ing these organisms with a net stretched across a small stream, he also col lected drifting aquatic forms. Several other investigations on drift were completed in the next two decades, leading to the conclusion that a continu ous drift of invertebrates must be considered a natural feature of streams (18, 19, 50, 59). Subsequent investigations, both in the United States and Europe, revealed the presence of large quantities of invertebrates in the drift (5, 42). But the contribution having perhaps the greatest impact in stimulating interest and research into the subject was published in 1954 by Miiller on the drift in north Swedish streams (74). In this work, Miiller also reported large quantities, even in a very small stream, and made obser vations on the qualitative relationships among drift, the bottom fauna, and