Anopheles (Myzomyia) farauti Lav. is recorded from the upper reaches of the Nevaka river, south-western Espiritu Santo, New Hebrides, to an altitude of 1,100 ft. and a distance of 12 miles from the coast. The presence of Plasmodium vivax and P. malariae in the blood of children from hill villages with no nearby Anopheline larval habitats is noted. These children, most of whom have never visited the coast, become exposed to infection during prawning trips to the valleys below their homes. The transmission of P. vivax and P. malariae is shown to take place in the interior of Guadalcanal, British Solomon Islands Protectorate, where a proven vector, A. (Myzomyia) punctulatus Dön., breeds at altitudes of up to at least 1,900 ft. above sea level. A. (Myzomyia) solomonis Belkin, Knight & Rozeboom, the status of which as a vector of Plasmodium is unknown, was found breeding at an altitude of 3,000 ft. Neither P. falciparum nor Wuchereria bancrofti were recorded from the hill districts visited. During the investigations several species of Culicines were collected. Features of the larval morphology of some of these are illustrated and briefly discussed in a systematic appendix.