The fresh water crustacean Daphnia magna is widely used as a test organism in aquatic toxicology to assess the adverse effects of individual substances or complex mixtures, e.g. industrial wastewaters. Cultures are held in several European testing laboratories and testing is typically carried out according to internationally standardised protocols. However, despite accounting for many potential confounding factors these guidelines do not currently take into account any specification related to the use of a specific clone. Cultures from seven laboratories were used to assess genetic variability by random-amplified polymorphic DNA polymerase chain reaction. Results pointed out the existence of two main clone clusters Responses in the acute Daphnia immobilisation test showed no direct correlation with genetic clusters resulting from random genetic markers (random-amplified polymorphic DNA) analysis. Considering that genetic differences are the most probable cause for the ecotoxicological test data, further analysis concerning gene expression and genetic stability should be performed.