Predictors of group size in Delphinoidea were investigated in a comparative study. Possible predictors included phylogenetic variables, variables of the physical environment, the diet, correlates of predation pressure and life-history parameters. The strongest predictors were the variables species and subfamily, which explained most of the observed variation in group size. Group size also increased with openness of the habitat and showed a U-shaped relationship with temperature. In conclusion, phylogeny seems to play the most crucial role in the evolution of group size in Delphinoidea. The simplest interpretation of this result is that group size resulted from a historical (random) process and has only been marginally shaped by direct selection.