Three sympatrically occurring members of a cichlid fish species complex from Lake Malawi
have previously been reported to show assortative mating in the field. Offspring from three
experimental tanks each housing both sexes of all three members of this complex were screened
at one to four microsatellite loci to determine paternity. Reproductive isolation was found
to persist without exception under laboratory conditions, providing strong evidence for
behavioural barriers to mating. Direct mate choice is suggested as the process preventing
interbreeding. The results are interpreted with reference to the rapid and explosive evolution
of the cichlid group.