We report a negative correlation between the weight of a female convict cichlid, Cichlasoma nigrofasciatum, and the magnitude of her defence against a model brood predator for a brood reduced to 100 eggs. We account for this relationship by the fact that a larger female is more likely to spawn a larger number of eggs than is a smaller female, making a fixed number of eggs relatively less valuable to the larger female. We performed two supplementary experiments that eliminated other logical explanations of the data. One experiment involved scaling the model brood predator. Results indicated that the size of model relative to the parent (over the range of ratios investigated) has no significant effect on magnitude of defence. The other experiment demonstrated no systematic trend in defence when brood number was unmanipulated. Thus, on average, larger females defended their naturally larger broods to the same extent as smaller females defended their smaller broods. Together, these three results suggest that female convict cichlids of different sizes do not value the same brood number equally. Copyright 1998 The Association for the study of Animal Behaviour.