The grass goby,Zosterisessor ophiocephalus, breeds in dark underground reproductive burrows excavated by the male in the intertidal muddy area of seagrass meadows. The present study under laboratory conditions, describes the behaviour exhibited by both male and female grass gobies during breeding. The most common behaviours displayed by the male involved physical contact with the female (e.g., prodding, tail beating), whereas the female mostly reacted by performing behaviours related to body and/or fin oscillations. The most frequent behavioural sequences performed were tail beating and prodding by the male, followed by female body arching and dorsal fin oscillations, and the latter followed in turn by male tail beating and prodding. Both sequences occurred with the highest frequency during courtship. A statistical analysis of the behavioural sequences indicated that the most significant ones were prodding by the male followed by female body arching, and the latter followed in turn by male prodding. Both sequences were performed with the highest significance during courtship. In addition, prodding the female was the most significant behaviour performed by the male both during courtship and spawning. Behaviours such as prodding and tail beating cause physical contact between the male and the female, whereas a female's body and fin oscillation induces water vibrations around the fish. Hence, behaviours related to a mechanical stimulation of the partner appear to play a fundamental role during breeding in the grass goby. Such behavioural patterns may have evolved because they are particularly effective in the grass goby's breeding environment, i.e. the dark burrows excavated in the muddy seagrass beds where the visual communicative channel results of little use.