During June and July, 1984, the ontogenetic changes in the composition of the natural diet of M. lunaris from Pallarenda beach, Townsville, Australia, were investigated by analyses of foregut contents using the percentage occurrence and percentage point methods. M. lunaris was found to be a predator of small crustaceans and molluscs and a facultative scavenger. There were marked changes in the composition of its diet during ontogeny: small individuals fed primarily on small crustaceans such as sergestids and copepods; large individuals fed primarily on anomurans and gastropods. The possible influence of ontogenetic changes in chelae function upon the diet is discussed.