Laboratory experiments were undertaken with Aedes aegypti infected with both Semliki Forest virus (SFV) from infant mice and Plasmodium gallinaceum from fowls to determine if such double infection of mosquitos suppressed their ability to transmit the malaria parasite, a possibility suggested to explain reduction in malaria transmission in Uganda in 1960 when Anopheles funestus and Anopheles gambiae were transmitting both malaria and o'nyongnyong virus to the African population. In general, transmission of fowl malaria was not prevented by SFV infection in Aedes, although some malariometric indices, and consistently the mean oocyst count, were slightly lower in doubly-infected mosquitos than in controls. In one experiment, however, 60% of the Aedes infected 8 days previously with SFV died within 48 hours of ingesting a malarious meal. Mortality was selectively in favour of the survival of SFV-infected mosquitos negative for, or least heavily infected with, malaria; depression in the presence of the virus of the intensity of malaria infection in the individual Aedes also occurred. Some physiological factor-a stress in adult life or, possibly more important, suboptimal larval nutrition-appears to have been crucial to eliciting the adverse effect on the mosquitos themselves and on their malaria infections. Suppression of the development of a malaria parasite in a mosquito, and of malaria transmission, by concurrent infection of the vector with an arbovirus can happen, but is by no means inevitable.It is shown that a doubly-infected Aedes can transmit both Semliki Forest virus and P. gallinaceum simultaneously.