The purpose of this research was to study the genetics of susceptibility and refractoriness of Anopheles gambiae Species A to Plasmodium species. Nine generations of selection resulted in a susceptible line (PB) showing 100% and a refractory line (LD) showing 0% susceptibility to Plasmodium berghel berghel (a rodent malaria). It was found that the parasite degenerated part way through the sporogonic cycle in the LD line. The F, progeny from reciprocal crosses between the lines differed 1n their susceptibility to the parasite. Backrrosses to the parent lines did not produce proportions of susceptible and refractory Individuals consistent with single gene Inheritance or with cytoplasmic Inheritance. Tests of the lines with two other species of rodent malaria, P. yoel11 and P.y.n1ger1ens1s gave similar results to those with P. b, berghel. In an attempt to check whether the genetic mechanism controlling susceptibility to P. berghei has an influence on human malaria Infection, the two lines were fed on a chimpanzee infected with P. vivax. The PB Une was fully susceptible to the human parasite while the LD line was partially susceptible. Feeding the two Unes of mosquitoes on human volunteers suffering from P. falciparum showed a difference between the lines in their rate of susceptibility to the Infection which was statistically significant 1n two out of four replicates. The results will be discussed 1n relation to the possible replacement of disease vectors by harmless strains.