The effects of grazing by the small oligochaete, Aeolosoma, on periphyton assemblages of different algal composition and on unconditioned detritus were examined. Natural water and periphyton inoculum were collected from a littoral environment and used to construct laboratory microcosms. Changes in algal community composition, microbial respiration, and structural indices (Chlorophyll a, protein, and carbohydrate) were measured in grazed and ungrazed cultures through time in 2 to 12 d experiments. Grazed cultures after 10 to 12 d had dissimilar algal community compositions in both the early and late summer periphytic assemblages as measured by Pinkham and Pearson's Index of Similarity (B). Changes resulted from stimulation (Green coccoids, thin green filaments) and suppression (certain diatoms) of individual algal taxa, and not from the dramatic shifts in comunity composition and structure effected by macroinvertebrate grazing. / Master of Science