Fourteen chemicals comprising alkylating agents, organotin compounds, an antimetabolite (aminopterin), and an antibiotic (porfiromycin) were evaluated as chemosterilants of Circulifer tenellus (Baker). Topical application and contact exposure were found unsatisfactory, but oral administration in sucrose solution through parafilm membranes was satisfactory and was employed throughout the study. Apholate and tepa were toxic at sterilizing dosages. Metepa, tepa, and Compound I ( N, N′ -hexamethelene bis-l-aziridinyl carboxamide) were the most effective sterilants; hempa was the least effective. Because of high sterilant activity and low toxicity, Compound I was selected as a model compound for detailed study. Male leafhoppers were somewhat more susceptible than females to the toxic as well as to sterilant action of Compound I. Susceptibility was highest in late-stage nymphs and it decreased in both sexes with advancing age. High degree of sterility was evident when matings were performed immediately after a 24-hour acquisition period, and it increased to a plateau at 48 hours, remaining constant for the entire 1-month period of observation. Females which mated to sterile males remained monogamous provided the 1st copulation was long enough (approximately 1.6 minutes) to insure passage of seminal fluid. Chemosterilized males, when confined at various ratios with normal males and females, were found to be sexually as competitive as nonsterile individuals, both under optimum environmental conditions and under temperature stress. Sterilization of the insect by 50Co irradiation was also demonstrated, 95% sterility having been obtained with doses of 16–20 krad for males and 4–6 krad for females.