From mid-November to late December 2011, we surveyed 12 sites in nine communes in and around the south-western part of the range of the Endangered crowned sifaka Propithecus coronatus in western Madagascar. We observed sifaka appearing to be P. coronatus at four sites in the Menabe Region; at three of them several of the sifaka were melanistic. Decken's sifaka P. deckenii were recorded at three sites, and Verreaux's sifaka P. verreauxi at one site. We found no evidence of sifakas between the Tsiribihina and Manambolo rivers west of latitude about 45°E, and local people claimed they have never existed there. We therefore recommend that the forests in this area be excluded from the current distribution range of P. coronatus. We found no sifakas resembling P. deckenii in the melanistic P. coronatus groups, which appears to confirm the hypothesis that most previous reports of possible sympatry between these two species can be better explained by melanism in P. deckenii. The typical individuals of P. coronatus we report showed lightly or heavily washed rufous coloration on the forearms and upper back. The melanistic forms we describe can be categorized as either a “very dark” form, characterized by dark brown to blackish coloration on the forearms and upper back, or as an “intermediate” form, showing dull rufous or light brown forearms and upper back. In reality there appeared to be a continuum in chromatic variation from the typically colored individuals, through the intermediate melanistic form, to the very dark form. These melanistic sifakas appear similar to the darker melanistic forms of P. deckenii north of the Manombolo River, rather than to the melanistic forms of P. coronatus reported to the south-east of our survey sites, or to melanistic forms of P. verreauxi south of the Tsiribihina River. Ancient gene-flow between sifaka taxa may be one of the causes of these melanistic tendencies, but we recommend further research to clarify the situation. These newly reported populations are at a high risk of local extinction. Containing melanistic forms not found elsewhere, we propose that they should be considered of considerable conservation importance with regard to preserving unique chromatic variation, and probably also genetic diversity, in P. coronatus.