We completed laboratory-feeding studies that demonstrated preference of the headwater stream fishes Gambusia holbrooki (Mosquitofish) and Clinostomus funduloides (Rosyside Dace) for smaller Gammarus pseudolimnaeus (amphipods) over larger ones. We also submerged oak leaf-litter bags in sections of streams with and without these fish predators. After three weeks, the mean number ± s.e. of amphipods per litter bag in streams with fish was significantly lower (289 ± 50 vs. 978 ± 122) and the average size of amphipods was significantly larger (13.9 ± 0.1 mg vs. 8.1 ± 0.1 mg), relative to streams without fish. The quantity and quality of leaf litter, however, were not significantly different. Top-down predation may have altered the population structure of stream-dwelling amphipods, but did not change leaf decomposition over the time of the study.