Diel variations in drift of periphytic diatoms were studied in a small 4th-order sandy stream and in enclosed recirculating water chambers in the laboratory to describe algal drift in the natural stream, identify algal species exhibiting diel periodicities in drift, determine if species-specific drift may be related to substrate selectivity, and determine if algal drift may be affected by benthic invertebrates. Stream water samples were collected at 2-h intervals for 24-h periods seven times from July 1986-July 1987. Algal drift ranged from 130-950 cells/mL and was dominated by the periphytic diatoms Achnanthes minutissima, Cocconeis placentula var. euglypta, Achnanthes lanceolata var. dubia, Achnanthes clevei, Amphora perpusilla, Cocconeis diminuta, and Fragilaria brevistriata. A small centric diatom, Cyclotella comensis, was also common. Diel periodicities in algal drift were not observed. Three laboratory studies were conducted in triplicate to study drift of algae attached to macrophytic plants (14-17 September), rocks (21-24 September), and sand (8-10 October). Photoperiod and temperature were adjusted to simulate conditions in the natural environment. Algal drift in chambers was collected at 2-h intervals. Two diatoms, A. minutissima and C. placentula var. euglypta exhibited diel periodicities in drift, but only in chambers containing macrophytic plants and rocks. No periodicities were apparent in chambers containing sand. Midday peaks in drift of both species averaged 6-15 times greater than minimum drift observed at night. After 2 d of normal photoperiod, we eliminated the next light period to monitor algal drift under 24 h constant darkness. Midday peaks in drift persisted indicating that diel periodicities in algal drift were not directly dependent on light availability. Algal drift was compared in chambers containing macrophytic plants with and without a common filter-feeding caddisfly, Brachycentrus americanus. Algal drift was not affected by B. americanus during the day; however, drift was greater in chambers with than without B. americanus at night. This was probably an effect of increased crawling and grazing activity and possible dislodgment of cells from substrates. Results of this study show that diel periodicities in algal drift are not readily evident in all small streams dominated by periphytic diatoms. Diel periodicities in algal drift are species-specific and under autogenic regulation which may be affected by light, substrate selectivity, and benthic invertebrates.