This paper describes the reproductive behaviour of Aidablennius sphynx, which was studied in the field during four breeding seasons. Artificial nest sites were used for the determination of nest contents. The breeding season started at the end of April, when water temperature had just reached 14-15 degrees C. The breeding season extended into August. Breeding males defended nests (crevices in the substrate), in which they cared for eggs laid by females. Female home ranges varied from narrow (about 10 m) to wide (about 45 m) stretches of shoreline. Females in narrow home ranges sometimes made excursions outside their home range. The number of males in nests that females could encounter was between 3 and 18. Individual males and females could have several sexual interactions per day with the same or different partners. Roughly 30% of the interactions led to nest entry by the female, and slightly less than 10% resulted in egg deposition by the female. Many different sequences of behaviour elements led to mating; for example male courtship display was not necessarily performed when mating resulted. Males def ended the same nest and received eggs continuously throughout the breeding season; there were no nesting cycles. Broods contained up to 7000 eggs from different females in different stages of development. Males did not receive eggs every day. If any eggs were received on a day, the average number was 280, laid by one or more females. Females in aquaria laid on average 286 (range 1-897) eggs at intervals of on average 2.5 (range 1-13) days. This is probably a too high estimate of female egg production, since the fishes in the aquaria were well fed. In the discussion it is argued that detailed descriptions of the breeding biology of individual fish species are relevant to theoretical considerations of reproductive styles in fishes.