An active biomonitoring study was carried out on the Algerian west coast using wild reference mussels (Mytilus galloprovincialis) sampled from the Kristel (K) site and transplanted in net cages during one month (between May and June 2007) to Oran Harbour (OH) and Mostaganem Harbour (MH), areas characterised by high levels of urban and industrial pollution. The biological response of the mussels was evaluated by their condition index and the use of a general stress biomarker (evaluation of lysosomal membrane stability: the neutral red retention time (NRRT) method), a genotoxic effects biomarker (determination of micronuclei (MN) frequency) and a neurotoxic effects biomarker (determination of the acetylcholinesterase (AChE) concentration). Compared to the K reference specimens, OH and MH caged mussels presented a significant decrease of NRRT in lysosomal haemocytes (56.45 š 26.48 min and 67.25 š 22.77 min, respectively) (78 š 16.97 min for K mussels), an MN frequency respectively 7.3 and 9 times higher in the haemocytes and the gill cells of the OH caged mussels, and 7.2 and 6.4 times higher in the two tissues of the MH caged mussels. Significant inhibition of AChE activity was noted in the gills (16.93 š 3.1 nmol min-1 mg prot-1) and the digestive gland (7.69 š 1.79 nmol min-1 mg prot-1) of the OH mussels, but only in the gills (23.21 š 5.94 nmol min-1 mg prot-1) of the MH mussels, compared to the organs of the K control specimens (35.9 š 6.4 nmol min-1 mg prot-1 in the gills and 11.17 š 0.49 nmol min-1 mg prot-1 in the digestive gland). This study reflects the interest in such in situ biomonitoring assays and the utility of these biomarkers for assessing the effects of pollution in the Algerian coastal marine environment.