A biosystematic study was made of cultures of closely similar species of Aphytis (Hymenoptera, Chalcidoidea, Aphelinidae) from various countries. Sexual isolation was an important barrier to interspecific hybridization. Laboratory manipulations were devised to partially overcome this. Some fertile interspecific hybrids were obtained and hybrid lines established. Fecundity and sex ratio of hybrids improved in successive laboratory generations. A mathematical index was developed, based upon extent of hybrid progeny production, sex ratio, and degree of fertility of F₁ hybrids, which depicts the degree of reproductive isolation between a given pair of species. It was concluded that the described species, africanus, lepidosaphes, melinus, fisheri and boloxanthus, are valid species with respect to each other and with respect to all other species studied. Aphytis coheni and “khunti,” A. lingnanensis and “2002,” and “R-65-23” and “2002” are considered to be semispecies with respect to each other. Additionally, the following are considered to be valid species with respect to each other: lingnanensis and coheni; lingnanensis and “khunti”; “2002” and coheni; “2002” and “khunti”; lingnanensis and “R-65-23”; “khunti” and “R-65-23”; and coheni and “R-65-23.”