Interspecific mating between male Cimex hemipterus and female C. lectularius bedbugs (Hemiptera: Cimicidae) occurs freely in the laboratory, even when many female C. hemipterus are available. Longevity and fertile egg production of female C. lectularius are reduced by mating with male C. hemipterus. In KwaZulu, South Africa, a correlation exists between the percentage of C. lectularius females which are interspecifically mated and the predominance of C. hemipterus in mixed domestic infestations of these bedbugs. When C. hemipterus constitutes over 75% of the infestation, the majority of C. lectularius females lay only sterile eggs.