A gelsolin-related protein was isolated from seminal vesicles of the annelid Lumbricus terrestris. Compared with the isoforms of the gelsolin-related protein previously found in the muscle of the annelid body wall, the isolated protein was assigned to the first isoform (EWAM-P1) because of its electrophoretic mobility, chromatographic elution behaviour, immunological cross-reactivity and identical nucleotide sequence of segments obtained by reverse transcription/polymerase chain reaction. Immunofluorescence studies with smear preparations of developing male germ cells revealed characteristic changes of the local distribution of actin and EWAM-P1 during spermatogenesis. These changes were correlated with the developmental transport processes and structural alterations. F-actin, as revealed by rhodamine-phalloidin staining, formed a toroid-shaped structure in cytoplasmic bridges connecting the germ cells to a central cytophore during the developmental stages. An actin antibody reacting with both G- and F-actin demonstrated that actin was concentrated at the proximal and distal parts of the spermatocytes. EWAM-P1 was also localized in these regions, with intense staining in the distal part of spermatocytes and young spermatids in which the Golgi complex and proacrosome resided. The anti-actin antibody further stained the periphery of the nucleus. This staining gradually reduced during sperm maturation and covered about half of the length of the nucleus in elongated spermatids. Co-localization of EWAM with actin implied a functional significance of this gelsolin-related protein for the rearrangement of the actin cytoskeleton during earthworm spermiogenesis.