The pattern of phenotypic and molecular variation within the polytypic olivaceous warbler H. pallida was examined. This species is distributed in the southern parts of the western Palaearctic, central Asia and in the arid parts of northern Africa, and also in parts of the sub-Saharan Sahel zone. Based on morphology, five subspecies, at times assigned to three groups, have been identified. By comparing morphological, behavioural, vocal and molecular variation, we investigated the phenotypic and phylogenetic relationships within the Hippolais pallida group. The morphological and genetic data of the present study support the view of splitting the olivaceous warbler into a western (former subspecies opaca) and an eastern form (former subspecies elaeica, pallida, reiseri and laeneni). Opaca is consistently and significantly larger than the other taxa in all size measurements, even if it does share morphological properties with elaeica in characters associated with migration. The song of the polytypic pallida differs clearly from that of opaca, and is remarkably consistent within its wide range which comprises four subspecies. The molecular analysis, and the resulting phylogenetic pattern, clearly separated opaca on a single branch distant from the other subspecies. The eastern form consists of two non-overlapping haplotype groups: elaeica with relatively diverse mtDNA variation, and the three African subspecies pallida, reiseri and laeneni, which all share the same or closely related mtDNA haplotypes. It remains open whether the African taxa should be regarded as three valid subspecies of the Eastern olivaceous warbler, or if they are better treated as a single African subspecies.