Although appearing in the literature as early as 1890, the brown lemur form Eulemur cinereiceps has recently resurfaced as a potentially valid taxon, distinct from neighboring, presumably closely related species such as white-collared lemurs (Eulemur albocollaris). We propose two scenarios for the potential separation of E. cinereiceps and E. albocollaris: (1) coastal and interior populations represent two distinct taxa and (2) the coastal population north of the Manampatrana River (the locality for purported museum specimens of E. cinereiceps) represents a distinct species from E. albocollaris found south of the river and in the interior escarpment forests. We tested these hypotheses using data from ground surveys and genetic sampling. Surveys were conducted in coastal forest fragments both north and south of the Manampatrana River in July–August 2006. Genetic samples were collected at two coastal sites and one interior forest. We used maximum parsimony, maximum likelihood, and neighbor-joining analyses on mitochondrial DNA regions to determine if populations from different sites clustered into diagnosable clades. Results from field surveys confirmed the presence of forms commonly referred to as E. albocollaris at the two southern coastal forests; no consistent phenotypic differences across sites were observed. All genetic analyses yielded identical results: coastal and interior populations do not cluster into separate groups, thus rejecting the first hypothesis. Eulemur species and all other day-active lemurs have apparently been extirpated from coastal forests north of the Manampatrana. Owing to the absence of lemurs from the northern coastal localities, we could not conclusively support or reject the second scenario. However, based on examination of the original plates and museum specimens, as well as the biogeographic patterns typical of this region, we strongly suspect that all populations from this area belong to a single species. We conclude with remarks regarding the apparent priority of E. cinereiceps for this taxon. Am. J. Primatol. 70:372–385, 2008. © 2007 Wiley-Liss, Inc.