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The effects of a single pulse, 21-d exposure of the commonly used herbicide alachlor (2-chloro-2′, 6′-diethyl-N-methoxymethyl acetanilide) on an algal community from a typical agricultural stream in Nebraska were studied using 18 stream microcosms located in a greenhouse, at six alachlor concentrations (0, 1, 10, 30, 100, and 1,000 μg/L). Effects of alachlor exposure at 1.0 μg/L were not significant (p < 0.05); however, at all other concentrations, alachlor had a significant negative effect on algal biomass. Differential taxonomic responses were observed, with approximately half the dominant algal taxa affected at levels >10 μg/L. Some taxa recovered from exposure by day 7, while others took longer or did not recover. A shift in the dominant algae was observed at higher concentrations (30, 100, 1,000 μg/L), and after 21 d these streams exhibited total algal cell densities significantly lower than the control and 1.0-μg/L-level streams. Results of this study suggest that alachlor inputs can alter both algal community composition and biomass in agricultural streams. Consequently, potential indirect impacts of alachlor on higher trophic levels warrant further investigation.