Conventional statistical methods of analyzing large-scale ecotoxicological field studies have centered around the use of univariate procedures, such as analysis of variance (ANOVA) and t testing. Although such techniques are well documented and robust, the measures produced reflect differences at the individual rather than the community level. Because such techniques permit only small subsets of the collated data to be analyzed at any one time, many analyses must be performed and interpreted. Multivariate analysis provides a viable and complementary alternative whereby a single analysis can incorporate all collated data and provide graphical and statistical measures of changes in community structure. In this study, the ordination techniques of redundancy analysis (RDA) and principal response curve (PRC) analysis were used to evaluate macroinvertebrate community responses to two pyrethroid insecticides in a replicated aquatic mesocosm. Observed changes in community structure between control and treated mesocosms were evaluated statistically using a before-after-control-impact (BACI) design Monte Carlo permutation test. Cypermethrin and lambda-cyhalothrin were applied at similar activity rates of 0.7 and 0.17 g active ingredients (a.i.)/ha, respectively. No significant effect was observed on the macroinvertebrate community, measured in emergence traps, for either pyrethroid at any rate. A significant change from the control community structure due to 0.7 g a.i./ha cypermethrin and 1.7 g a.i./ha lambda-cyhalothrin application was identified in the macroinvertebrate communities in artificial substrate samples, principally due to a decline in Gammaridae and Asellidae. The Asellidae population that was treated with cypermethrin recovered fully toward the end of the study period. The capability of multivariate techniques to draw out trends at a major community level within a large data matrix, combined with their increased ecological relevance compared to studies at lower levels of biological organization, makes them powerful analytical tools. Increased integration of multivariate techniques into regulatory assessments is likely to improve the value and efficiency of ecotoxicological field studies for ecological risk assessment.