ABSTRACT: The distribution and seasonality of humpback whales Megaptera novaeangliae in the southwest Indian Ocean are investigated from a number of sources. These include sighting and acoustic data collected on shore-based surveys on the African coast andfrom dedicated ship-based surveys in the coastal waters of Mozambique, Madagascar, across the Mozambique Channel and along the Madagascar Ridge, plus aerial sighting data from the Durban (South Africa) whaling ground, historical catch data by 10 d periodfor whaling operations at Durban, Linga Linga (Mozambique) and Madagascar, and incidental sightings data from a research ship and transiting merchant vessel. Three principal migratory routes are proposed. The first ('East African') strikes and leaves theAfrican coast at least as far west as Knysna (23° E). The number and timing of peak abundances at Durban, Cape Vidal (South Africa) and Linga Linga suggest that this migration continues as far as central Mozambique, where abundance peaked in the first 10d of August, and is slower and more protracted on the way south than on the way north. The second route ('Madagascar Ridge') supplies the coast of Madagascar directly from the south, probably via the Madagascar Ridge. The seasonality of catches in modernwhaling in southern Madagascar implies that this migration proceeds somewhat further north, and one nineteenth-century whaling ground was on the northeastern coast of Madagascar at 15°30'S. The existence of the third migratory route ('Central MozambiqueChannel') is based principally on the discovery of singing humpback whales in the centre of the Mozambique Channel in July/August and September, separate from contemporary singers on the continental shelves of Mozambique and Madagascar. The winterdestination of this route may be the Comores Islands and Aldabra, where there have been a number of recent sightings.