none The genetic variation of sex ratio and sex allocation were examined in a series of half-sib analyses on the sex ratio of braconid parasitoid wasp Heterospilus prosopidis populations collected in Hawaii and Arizona. The mean threshold value and the range of the threshold for change in the sex of offspring in response to resource quality (host size) were determined. Estimates of the narrow-sense heritability (h2) of sex ratio at a specific host size ranged from 0.185 to 0.315, and those of the sex changing point (threshold value) ranged from 0.220 to 0.342. The coefficient of variation (CVA) of sex ratio was significantly larger than CVA of body weight. We discuss factors that maintained the significant additive genetic variation of sex ratio. Genetics; Evolution; Ecology; Population dynamics; Population structure; Land zones; Nearctic region; USA; North America; Oceanic islands; Pacific Ocean islands Heterospilus prosopidis (Braconidae); Sex determination genetics; Arizona & Hawaii; Variation; Population sex ratio; Arizona; Hawaii; Genetic variation of sex ratio & sex allocation