Microcosms were used to assess the impact of chromium on free-living marine nematodes. Nematodes were exposed to three chromium concentrations (500 ppm (dm, dry mass), 800 ppm (dm) and 1,300 ppm (dm)), and effects were examined after 4 weeks. Results showed significant differences between univariate measures of control nematodes and those from medium- and high-chromium microcosms. Most, decreased significantly with increasing level of chromium contamination. The medium-chromium treatment seems to be the minimal concentration that could has a negative effect on nematodes. Results from multivariate analyses demonstrated that responses of nematode species to chromium treatments were varied: Leptonemella aphanothecae was eliminated at all doses tested and seemed to be intolerant species to chromium; Daptonema normandicum and Sabatieria longisetosa which significantly increased at 500 ppm chromium (dm) appeared to be "opportunistic" species at this dose whereas the two Bathylaimus species (Bathylaimus capacosus and Bathylaimus tenuicaudatus) which increased at all the doses tested seemed to be "chromium resistant". As we think such "opportunistic" and "resistant" species could be used as sensitive indicators of unsafe marine food. The use of microcosms has allowed the effects of the chromium on nematodes to be assessed individually, which was not possible in the field.