Allometric variation in testis length was used to test the prediction that the tests of males of raptor species with non-reversed dimorphism in size (NRSD; males larger sex) are proportionally smaller than those of males of species with reversed dimorphism in size (RSD; females larger). Testis length-body weight relationships of 17 species of mainly Australian raptors were analysed. After removal of body weight as a variable, testes of males of NRSD raptors were significantly smaller than those of RSD males. No relationship with degree of dimorphism was detected. These findings accord with the hypothesis that in the dimorphic raptors (both NRSD and RSD) it is the larger sex that has significantly diverged in size (through sexual selection). That the males of NRSD raptors have increased in size without an equivalent increase in testis size points to a possible difference in their breeding tactics from RSD raptors. It might be expected that, unlike RSD males, these large males guard their females to lessen the chance of promiscuous mating and cuckoldry, and that they copulate less frequently than the males of RSD species.