The main objective of this study was to investigate two predictions of sexual selection theory concerning interspecific variation in testis size among strepsirhine primates (Lemuriformes and Lorisiformes). First, the unique evolutionary history of lemurs provides an opportunity for an independent test of the predictions of sperm competition theory regarding the relationship between mating system and relative testis size. Second, I examined the evolutionary relationship between the morphological correlates of pre- and postcopulatory competition (i.e., between sexual dimorphism and testis size) because polygamous lemurs, in contrast to other polygamous primates, lack sexual dimorphism. Based on measurements from 174 captive strepsirhines from 24 species, I found that multi-male species had significantly larger testes than pair-living ones, but that they did not differ significantly from solitary species. This result deviates from theoretical expectations, but may be the result of yet-unknown heterogeneity in mechanisms of male-male competition in both multi-male and solitary species. There was no difference in relative testis size between nonmonogamous lemurs and lorises, indicating that presumably lower levels of precopulatory competition are not necessarily compensated by more intense sperm competition. Body size and phylogenetic effects were also found to considerably affect interspecific variability in testis size. Analyses of independent contrasts revealed that evolutionary changes in mating system, testis size, sexual size, and canine dimorphism were not, or only weakly, associated in this monophyletic group of primates. Additional comprehensive comparative studies of sexual dimorphism, testis size, mating system, and copulatory behavior in these and other taxa are indicated to illuminate general patterns and causes of covariation among these traits