In this study, we sequenced a partial segment of the mitochondrial control region from 21 proboscis monkeys of the Klias peninsula, the last large population remaining on the west coast of Sabah, Malaysia. Our results showed that this population retains substantial genetic variation, and subpopulations from different river systems in the central and southern portions of the Klias share multiple haplotypes. We also compared our data with previously generated sequences from 2 eastern populations of proboscis monkeys in Sabah and found little evidence of regional genetic structure. Based on these results, we argue that conservation efforts should focus on restoring connectivity between central and southern Klias peninsula proboscis monkeys and discuss future analyses needed to better understand the mitochondrial structure of proboscis monkeys in Sabah. ... : MunshiSouth.JOH.Nasalis.HaplotypesThis file includes mitochondrial d-loop haplotypes and haplotype frequencies for four populations of proboscis monkeys from Sabah, Malaysia (NE Borneo). The haplotype file is in Arlequin format. Original sequences are available on Genbank. ...