To investigate whether male common gobies, Pomatoschistus microps (Pisces, Gobiidae), treat their offspring differently depending on confidence of paternity, we conducted an experiment in which randomly chosen males either spawned alone with a female, or with a sneaking male present. Males did not treat their brood differently whether they had experienced sneaking or not. Our estimates of parental care, nest defence against potential egg predators and fanning rate were the same for the two treatments. Furthermore, there was no difference in filial cannibalism (eating their own progeny) between males that had been sneaked upon and males that had not. However, nest-guarding males that ate some of their brood had a smaller original brood area than other males. This suggests either an increase in paternal expenditure with increased brood size or a threshold value (absolute brood size or proportion of nest space covered) above which males do not cannibalize eggs. (C) 1998 The Association for the Study of Animal Behaviour.