Introgression due to hybridization is important for our understanding of species divergence. In this study, we analyze potential introgression between Macaca mulatta and M fascicularis using DNA sequences from 19 genes. While the putative introgression between these species has been studied previously, it has not been analyzed using nuclear loci from multiple chromosomes. Phylogenetic analyses of these genes revealed shared variation at several of the loci. Neutrality tests indicated balancing selection did not fully account for these shared polymorphisms. Testing the null model of Isolation with Migration (IM) indicated interspecific gene flow caused the shared variation, revealing significant gene flow from M mulatta into M fascicularis (2Nm=l.23) and very limited gene flow from M fascicularis into M mulatta (2Nm=0.07). This asyrtlmetry is consistent with previously reported differences in male aggression between these species. Genome-enabled detection of hybridization in these primates opens new avenues of research in genome evolution and speciation.