The applicability of terrestrial black slugs Arion ater (Mollusca, Gastropoda) was studied for biomonitoring environmental exposure to polycyclic aromatic hydrocarbons (PAHs). In laboratory experiments, slugs were orally exposed to benzo[a]pyrene (BaP) for a short term (3 days) or a long term (119 days) period. Test animals were collected in the field, or were reared under laboratory conditions to ensure that they had no history of PAH-exposure. Benzo[a]pyrene hydroxylase (BPH) activity was measured in the digestive gland as a biomarker for BaP exposure. Bulky DNA adduct formation in kidney was measured as an effect biomarker for BaP bioactivation into DNA-binding metabolites. Although success of clutching was relatively low (5 out of 18 slugs produced egg packages), sufficient number of slugs were obtained to perform exposure experiments due to high hatching (89%) and survival rates (79%). After a short exposure to a relatively high BaP doses of 20 and 200 microg/g fresh feed, a dose-dependent and significant increase of BPH activity and bulky DNA adduct levels could be demonstrated in A. ater. Induction factors were low (two times control level), but optimization of the test conditions yielded a higher BPH induction factor of 4.8 times control level. BPH activity and bulky DNA adduct levels, however, did not increase after a long-term exposure to environmentally relevant BaP doses (upto 0.25 microg/g fresh feed). Based on this lack of response after realistic exposure it is concluded that A. ater is not sensitive to BaP exposure and, therefore, not suitable for monitoring environmental exposure to PAHs.