Sexual isolation was studied in several species and semispecies of Aphytis, aphelinid parasites of diaspine scale insects. The multiple-choice technique was used in this study, and coefficients of isolation and joint isolation were derived by using previously developed formulas. These data, in combination with other information obtained from experimental hybridization, were used to clarify the taxonomic relationships among the different species and semispecies. We have attempted to define “semispecies” in more precise terms than those previously used. Our objective was to help determine what the degree of partial reproductive isolation between two populations must be in order that they may be called semispecies with respect to each other. To achieve this, we used the coefficient of joint isolation.