In order to define evolutionarily significant and management units (ESUs and MUs) among subpopulations of Sumatran (Pongo pygmaeus abelii) and Bornean (P. p. pygmaeus) orangutans we determined their genetic relationships. We analyzed partial sequences of four mitochondrial genes and nine autosomal microsatellite loci of 70 orangutans to test two hypotheses regarding the population structure within Borneo and the genetic distinction between Bornean and Sumatran orangutans. Our data show Bornean orangutans consist of two genetic clusters-the western and eastern clades. Each taxon exhibits relatively distinct mtDNA and nuclear genetic distributions that are likely attributable to genetic drift. These groups, however, do not warrant designations as separate conservation MUs because they demonstrate no demographic independence and only moderate genetic differentiation. Our findings also indicate relatively high levels of overall genetic diversity within Borneo, suggesting that observed habitat fragmentation and erosion during the last three decades had limited influence on genetic variability. Because the mtDNA of Bornean and Sumatran orangutans are not strictly reciprocally monophyletic, we recommend treating these populations as separate MUs and discontinuing inter-island translocation of animals unless absolutely necessary.