During a zoological survey in north-west Madagascar (Sahamalaza Peninsula) we documented aspects of the natural history of Boophis occidentalis (formerly a subspecies of B. albilabris and here raised to species level). Individual age assessed by skeletochronology ranged from 4 to 11 years. Breeding behaviour was observed at a seasonal stream after heavy rainfalls: five choruses of eight to 90 males aggregated in shallow brook sections. The males emitted low frequency calls and engaged in scramble battles. Only two females were found. A couple laid a large number of eggs, attached as a single layer on submerged stones. Tadpoles reared from these eggs had the typical morphology of brook-breeding Boophis species with a 1:5+5/3 keratodont formula and a relatively flattened body. Adult males had an anteroventrally oriented cloaca, a morphological trait shared with B. albilabris. Advertisement calls were similar in general structure to those of B. albilabris recorded at Anjanaharibe-Sud. They were unharmonious and consisted of pulsed notes emitted at irregular intervals. Much lower pulse rates in B. albilabris (30–38 pulses/s) than in B. occidentalis (90–109 pulses/s) support the specific distinctness of these taxa. The combination of characters related to reproductive biology in B. occidentalis and B. albilabris is unique in Boophis. The acontinuous breeding behaviour and large number of eggs per clutch are traits found in the pond-breeding species of the B. tephraeomystax group, while larval morphology and habitat are characteristic of brook-breeding species. Available morphological and molecular data place B. albilabris and B. occidentalis into the brook-breeding Boophis radiation. The acontinuous timing of reproduction and large egg number may be secondary adaptations that allowed the species to colonize arid regions of western Madagascar.