Microcosms were used to assess the impact of permethrin on the abundance and diversity of free-living marine nematodes. The nematodes were exposed to three permethrin concentrations (50, 100 and 150 μg l–1), and the effects were examined after 25 days. The abundances of nematodes at all permethrin concentrations significantly exceeded those in the controls. Multivariate analyses demonstrated that responses of nematode species to permethrin treatments were varied: Pselionema sp., Prochromadorella neapolitana and Spirinia gerlachi were eliminated at the low dose and seemed to be species intolerant to permethrin; Trichotheristus mirabilis and Xyala striata, which increased with increasing contamination levels, seemed to be 'opportunistic' and/or 'resistant' species. Results showed significant differences between univariate measures of diversity of control nematodes and those from permethrin- contaminated microcosms, where all decreased significantly with increasing abundance of the most tolerant species to permethrin. The use of microcosms has allowed the effects of permethrin on nematodes to be assessed individually, which is not possible in the field.