In the absence of any treatment in the Geremeas valley of Sardinia there was no build-up of the population of Anopheles l. labranchiae either as larvae in the vicinity of the original focus, or as adults in human or domestic animal shelters. There was no significant diffusion of labranchiae from the immediate area of the original positive to other suitable breeding places in the valley. The invasion of former labranchiae breeding places by Anopheles hispaniola and the replacement of the former by the latter was demonstrated.
The factors which may have contributed to the replacement of labranchiae by hispaniola are discussed, and it is suggested that the present residual population of labranchiae may differ in essential behavior characteristics which would limit its effectiveness as a malaria vector, but the study was not conducted over a long enough period to establish this with certainty.
On leave of absence from the Gorgas Memorial Laboratory, Panama.