Inheritance of gregarious or solitary oviposition in Muscidifurax raptorellus Kogan and Legner involves two phases: (1) the female parasitoid expresses either trait shortly after mating, the magnitude of which is apparently determined by the male genome; and (2) the trait is then apparently fixed into a genome of her offspring who demonstrate it in the virgin state. In the first phase, less than one-half the magnitude of behavioral expression is shown, full expression occurring in the F1 diploid virgin hybrid. In matings with hybrids produced from subsequent backcrosses, more than one-half of the behavior is expressed shortly after mating. Traits related to gregarious oviposition (number parasitoids developed per host and total progeny) were also similarly initially expressed and subsequently inherited. A polygenic mode of inheritance is apparent, with ≥3-22 genes estimated. The extranuclear influences prior to inheritance may involve microorganisms and/or chemicals (enzymes) present in hymenopteran seminal fluid. Natural selection in this system should be accelerated by males causing the expression of a portion of the quantity of heritable behavior within the same generation.