When a batch of young of the mouthbreeding cichlid Haplochromis burtoni is divided into two groups of equal size, they show different behaviour. The ones that are caught first are less aggressive than the second, more shy ones. If agonistic behaviour has a positive selection value, it can be an important factor for the rapid speciation in cichlid fishes. In order to investigate this assumption experimentally, one has to bear in mind that early experience, group size and group density can influence the agonistic behaviour.