We tested the effects of four concentrations (0.2, 1.0, 4.0, and 20.0 μg/L) of the organophosphorus insecticide azinphosmethyl on natural zooplankton communities using littoral ecosystem enclosures. Individual taxa and major zooplankton groups (cladocerans, copepods, and rotifers) exhibited concentration-dependent responses, with most of the significant effects at the two highest concentrations. Analyses of population abundance were more sensitive to the effects of azinphos-methyl than were analyses of major zooplankton groups. Principal component analysis of communities differentiated the contributions of cladocerans, copepods, and rotifers to total community variance. The highest principal components were associated with cladocerans, the group also identified by univariate statistics to be most sensitive to azinphos-methyl. Ordination using the first two principal components depicted the multivariate status of the zooplankton communities through time and showed concentration-dependent effects and recovery from the pesticide at the community level. Principal component ordination was a more sensitive metric of community-level effects than was taxon richness. Communities in the highest treatment concentration had not fully recovered by the last sampling date, 78 d after treatment.