A test for X chromosome involvement in causing hybrid male sterility was carried out with X chromosomes which in the salivary glands have distinctive polytene banding patterns designated as types Xa and Xb. Anopheles dirus species A and species B were mated with each other and the F1 females backcrossed to each parental population. The males from the backcross to species B were found to include a significantly higher proportion which could inseminate than those from the other backcross. A difference in this direction is unexpected on the X-Y interaction hypothesis of the causation of the sterility. The results rule out such a hypothesis but suggest that the X chromosome may play a contributing role in causing sterility.