The objectives of this study were to describe temporal variations in the downstream transport of periphytic algae, referred to as algal drift, in a small stream. It was hypothesized that algae in the drift exhibited a diel periodicity in their detachment from and re-colonization onto the streambed. Diel variations in algal drift composition and abundance in the natural stream were described for each of several discrete sampling dates selected over a one year period. Possible effects of substrate type, photoperiod, and disturbance by benthic invertebrates on species-specific algal drift activity were examined using experimental re-circulating water chambers as artificial streams in the laboratory. Statistically significant patterns in algal drift were identified using time-series analysis. Independent and interactive effects of light availability and invertebrate activity on algal drift were determined using two-analysis of variance. Diel periodicities in algal drift were species-specific and independent of photoperiod; however, significant differences in drift activity were observed between experimental chambers with and without invertebrates. These results suggested that regulatory mechanisms underlying diel patterns in algal drift were endogenous, but influenced in part by exogenous factors.