Tidal freshwater fish assemblages were sampled quantitatively from beds ofHydrilla verticillatain the Potomac River, Maryland, to assess local impacts of mechanical plant harvesting. Seasonal and diel estimates of fish density, biomass and species richness were compared at an undisturbed site and an adjacent mechanically harvested site. Harvesting had a “pruning” effect onHydrilla: plant biomass levels at the harvested site exceeded those at the undisturbed site after periods > 23 d. Fish species composition at the two sites was very similar and species richness differences were minor. Significantly lower (P<0.05) mean fish density and biomass values were found at the harvested versus the undisturbed site < 23 d after plant removal. However, 43 d after plant removal, fish density was significantly higher (P<0.05) at the harvested site, while biomass differences were minor. Species-specific differences suggested that harvesting improved habitat for pelagic species (e.g.,Menidia beryllina) but negatively affected cover-oriented species (e.g.,Fundulus diaphanus). Results suggest impacts of mechanical harvesting on the fish assemblages investigated were short-term and minor at the local, community level. However, ten species were killed in the mechanical harvesting process. We estimated that this immediate loss represented 11–22% of fish numbers and 4–23% of biomass. Mechanical harvesting, when macrophyte beds are in short supply, may not be prudent.