The acute copper sensitivity of 44 European freshwater cladocerans, from four families (Daphniidae, Bosminidae, Macrothricidae, Chydoridae) and 13 genera (Daphnia, Ctenodaphnia, Ceriodaphnia, Simocephalus, Scapholeberis, Bosmina, Acantholeberis, Alona, Acroperus, Chydorus, Eurycercus, Disparalona and Pleuroxus) were assayed. The 48-h EC(50)s of field-collected organisms tested in reconstituted standard laboratory water ranged from 5.3 to 70.6 mug Cu L(-1). Only among Ctenodaphnia were significant intra-species differences observed. Significant inter-species differences were noted among Alonina and Daphnia. Between all genera tested, a maximum of a 12-fold difference in copper sensitivity was noted. Most animals were more sensitive than a laboratory D. magna clone. A weak non-significant increasing trend was noted between mean cladoceran 48-h EC(50) and ambient copper concentration of the different aquatic systems, suggesting acclimation/adaptation in the field. A positive relationship was also observed between the 48-h EC(50) of the field-collected cladoceran species (without the Chydoridae family) and the size of the organisms.