Field investigations and laboratory experiments were performed to test the applicability of the intersex phenomenon in Littorina littorea for biological TBT-effect monitoring and to give further insights into toxicological aspects of intersex development. The intensity of the phenomenon is correlated with the degree of TBT contamination in the environment, Statistically highly significant correlations of biological indices and TBT concentrations in tissue and sediment exist, Inter sex can be induced only in juvenile and sexually immature females and its intensity depends on the ontogenetic stage of development during TBT exposure, For the first time, malformations in male periwinkles closely related to the TBT contamination can be described; the reduction of male mamilliform penial glands shows highly significant correlations to TBT concentrations in tissues. The inter sex phenomenon in Littorina littorea is proposed for a biological TBT-effect monitoring in those areas where the dogwhelk Nucella lapillus, as the more sensitive species in European survey's, is absent. The intersex index (ISl) as the mean value of the intersex stages in a sample should he used as the most sensitive biological parameter fbr the assessment of the TBT contamination in Littorina littorea. (C) 1997 Elsevier Science Ltd.