Nematodes are very abundant and fundamental group of soil organisms and non-parasitic species are important for soil quality. Therefore, nematodes are suitable as bioindicators of soil conditions and as organisms for toxicity testing. Nematodes used in laboratory toxicity tests are comparable to other soil species. Moreover, they are often referred as less demanding to place and time and as more sensitive. For our studies we used Caenorhabditis elegans strain N2, var. Bristol. Tested chemicals were 4 congeners of aza-PAHs, chlorinated paraffin and 2 organochlorinated pesticides in soil, aquatic media and agar. Interpretation was determined as mortality after 24 and 48 hours. Then were assessed LC50 value, compared results from 24/48 hrs test and different test media. In soil is LC50 value after 48 hrs higher due to sorption in soil. In aquatic media and agar after 48 hrs the LC50 value decreases. Agar is a specific medium, tested chemicals respond otherwise than in soil or aquatic medium and therefore results from agar test can be compared only with agar test data. This research was supported by Grant Agency of Czech Republic project 525/04/P159.