Melanistic leopards Panthera pardus are common in south-east Asian forests but the exact frequency of this variant phenotype is difficult to assess. Records from camera-trapping studies conducted at 22 locations in Peninsular Malaysia and southern Thailand between 1996 and 2009 show that only melanistic leopards were present in samples south of the Isthmus of Kra. During 42 565 trap-nights, we collected 445 photos of melanistic leopards and 29 photos of the spotted or non-melanistic morph. All 29 photos of spotted leopards came from study sites north of the Isthmus. These results indicate that this recessive trait may be nearly fixed in P. pardus populations of the Malay Peninsula, suggesting a unique evolutionary history of leopards in the region. Assuming a very small effective population size (Ne=100) and a high initial allelic frequency, at least 1000 years would be expected to elapse until a neutral allele became fixed. The severe bottleneck implied by this scenario provides a testable hypothesis that can be addressed using molecular markers and evidence of past glacioeustatic changes across the region. Although natural selection might lead to rapid fixation of melanism within Malayan leopards, had their effective population size been much larger (e.g. Ne=5000) and stable, with a lower allelic frequency, the fixation would require a longer time span (e.g. 20 000 years) if induced by genetic drift alone.