The observation in Kenya that Aëdes (Skusea) pembaensis (Theo.) associates with the crab, Sesarma meinerti, was duplicated in Lumbo, on the northern coast of Mozambique. Eggs were seen abundantly on crabs inhabiting certain colonies among mangroves at the edge of a tidal estuary. Females of A. pembaensis were caught in gradually decreasing abundance at collecting sites progressively further inland from the head of the estuary, while males were almost totally confined to the breeding area. Swarms of males were not seen, and it was suspected that mating took place within crab burrows.

Populations of A. pembaensis in 1959 and 1960 were approximately equivalent at Lumbo, whereas some other mosquito species with fresh-water larval stages were reduced in 1960 owing to failure of the rains. The estuarine environment, regulated by the tides, is apparently more stable than are terrestrial habitats. Cycles of A. pembaensis may therefore be less variable than among dry-land forms.

Females of A. pembaensis disturbed at a crab colony were found to bite spontaneously in the day-time. Captured specimens showed a marked discrimination in biting preference for one human out of three tested. The preferred individual experienced a severe reaction to the bites, while the other two had none whatsoever.

Precipitin tests indicated human beings as the only source of blood in A. pembaensis, although the method of collection probably lends bias to this observation. Eight strains of an apparently new virus were isolated from females of A. pembaensis in 1959 and four in 1960. In addition, two strains of a virus of the Bunyamwera group were isolated in 1960. The source of these viruses and their significance in A. pembaensis require further investigation.