When Anopheles gambiae males were allowed to mate with Anopheles melas females (X-2), hybrids were produced. The sex ratio was 23 males to one female and the hatch was fairly low (22.9%). In the reciprocal cross (X-6) the hatch was greater (51.6%) and the sex ratio approximately 1:1. In each cross the per cent of larvae pupating was about the same and the males appeared to be sterile.
One of the hybrid colonies, X-4 (X-6 females and A. gambiae males), was inbred for three filial generations during which the per cent of eggs hatching increased from 5.3 to 83.3, thus establishing a hybrid strain. When the males of these third generation hybrids were back-crossed to A. gambiae females the results were normal, but when back-crossed to A. melas females the progeny were predominantly male, as in X-2, the only other group containing female A. melas heritage.