| Sumario: | Morphological and genetic variation is evaluated among populations of the bat, Eidolon helvum, in the islands of the Gulf of Guinea (Central Africa). The populations from the islands of Bioko, Pr´ıncipe, and Sa˜ o Tome´ do not show significant phenetic differentiation, although a trend towards a reduction of size is found in the latter two islands. The low genetic distances between populations, as well as their values of Wright’s fixation indexes, suggest that gene flow has hampered differentiation on these islands. In contrast, the population from Annobo´ n, the smallest and farthermost island, shows remarkable morphological and genetic differentiation. On the mainland, E. helvum displays unique migratory and dispersal behaviours, but migratory behaviour was not found in any of the island populations. The combination of selective forces in harsher oceanic environments and restricted gene flow among populations appears to have favoured the high degree of morphological differentiation of E. helvum on Annobo´ n. Due to the extended length of the dry season in Annobo´ n, an earlier achievement of sexual maturity—and consequently smaller size—may be advantageous in the absence of migration. The differentiation is more marked among females, which also suggests that selection may be linked to the reproductive pattern. The population of the island of Annobo´ n is herein described as a new subspecies, Eidolon helvum annobonensis subsp. nov.
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