Ooencyrtus submetallicus, an egg parasite of Nezara viridula (L.), reproduces by thelytokous parthenogenesis. The males are functionless. Gynandromorphs occur: these are basically female with male areas of variable extent in the anterior portion of the body. The males and gynandromorphs are usually rare but can be produced at will in considerable numbers under particular conditions. Development in O. submetallicus occurs within a temperature range of 60-90ºF. Reproduction is slow at 70ºF or lower and rapid at 8OºF, whilst temperatures above 85ºF cause considerable mortality in the immature stages. Sex is determined by the temperatures to which a parent female is exposed during development and during adult life. 85ºF is the critical sex-determining temperature. Lower temperatures produce female progeny, temperatures of 85ºF or higher produce male progeny. Gynandromorphs evidently arise when the parent female, at a critical stage of egg development, is exposed successively to female-producing and male-producing temperatures. The degree of maleness of the gynandromorph is apparently dependent on the stage of egg development at which this change of temperature occurs. Ability to reproduce is retained only by weakly male gynandromorphs. The sex of the eggs deposited during the first few days of adult life is determined by pre-emergence temperatures: the sex of eggs laid subsequently may be largely determined by post-emergence temperatures. High (male-producing) pre-emergence temperatures appear to have a more profound and persistent effect on sex of the progeny produced subsequently than do low (female-producing) pre-emergence temperatures.