The present paper is the first of two dealing with the seasonal distribution, hourly variation in biting activity, and vertical distribution of certain Southern Nigerian mosquitos. The technique employed in the studies under discussion resembled that used by Haddow (1947) in Uganda. Human bait was used in both cases and the agreement between the results obtained in the two localities is felt to be such as fully to justify the employment of such methods. Over fifty species of mosquitos were taken in all and fourteen of these proved to be sufficiently abundant to yield significant figures. Four of these are discussed and the remainder will be dealt with in the second paper.

The most remarkable feature of the seasonal distribution was the very restricted period during which Anopheles gambiae was present in any abundance. This species occurred in large numbers only during the heavy rains of June and July and, to a less extent, in association with the small rains of September–October. During the dry season it almost disappeared. Aëedes africanus showed a similar distribution to that of gambiae but Taeniorhynchus africanus and Anopheles hargreavesi were most numerous during the dry season. It is suggested that in the area under consideration the latter species may well be a more important malaria vector than gambiae owing to its greater abundance and more uniform seasonal distribution.