Hydrobia ulvae specimens collected from 16 stations along the German North Sea and Baltic coasts exhibited imposex (occurrence of male parts in addition to the female genital system). For the purposes of comparison, a description of both the male and unaffected female genital systems is presented. Four different imposex stages with two types of development were identified and documented with scanning electron micrographs for the first time. The percentage of imposex-affected females (an average over all the localities sampled) was about 44.3%, and 12.9% were definitively sterilized. The phenomenon of sex change was not observed. The vas deferens sequence index, imposex incidence, percentage of sterilized females and the average female penis length are recommended as biological effect monitoring parameters in response to tributyltin pollution.