We analyzed six organochlorine compounds (HCB, lindane, p,p'-DDE, PCB # 138, #153, and # 180) from populations of the European great tit (Parus major) and from five species of bats (Pipistrellus pipistrellus, Nyctalus noctula, Plecotus auritus, Plecotus austriacus, Myotis myotis). A weak positive correlation between Kow (n-octanol /water partition coefficient) and bioaccumulation (measured as residue values) was found within each species. However, the ratio of the concentrations in bats compared to titmice increased significantly with increasing Kow. We argue that this is explained by the intensive juvenile feeding through long-term lactation and thus by the more intensive transfer of strongly lipophilic xenobiotics from mother to juvenile in mammals compared to birds. Further, there were clear variations between bat species with ranges of up to 11 times the lowest value for a single compound. These differences are related to different feeding habits of the adults and to different species-specific life history traits, such as age at maturity and number of offspring.