Populations of sculpin (Cottus cognatus) were altered to create stream sites with increased and decreased fish density. Subsequently, the response of benthic insect species was observed. Sculpin predation appeared to have little or no effect on size distributions and secondary production of most benthic invertebrates. However, densities of some trophic groups increased by two- or three-fold following reductions of sculpin densities. Within sites of sculpin removal, densities of gatherer/scrapers were twice those of addition or reference sites. Densities of total insect populations in reduction or reference sites were two to three times greater than those in the addition sites. Several trophic groups, particularly scraper/gatherers, showed the highest densities in sites of unaltered sculpin densities. Filter-feeders presented a third response to the manpulated sculpin populations; their densities increased with increased sculpin density. This suggests a negative sculpin influence on competitors of filterers.