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The genetic basis as well as the mode of inheritance of polyandry in Spodoptera exigua Hübner was studied in the laboratory by using a simple divergent selection experiment followed by F1 reciprocal crosses, F2 and backcrosses. There was an effective response to artificial selection for high (H line) and low (L line) female mating frequency with significant separation of the lines by the second generation of selection. The mean female mating frequency in the parental generation (1.57 matings per female) reached plateaus of 2.50 and 1.25 matings per female in the H and L lines, respectively, after six generations of selection. Selection response becomes saturated at about 90% and 25% levels of polyandry (percentage females re-mating) in the H and L lines, respectively, and consequently mono- and polyandric pure strains were not obtained. Polyandry levels in offspring from the H and L lines and their hybrids in F1, F2 and backcrosses consistently indicate that female mating frequency was more or less proportional to the relative amounts of genes derived from the H and L lines. Such a clear pattern of hybrid responses, together with the gradual selective changes under artificial selection, suggests the involvement of a polygenic system. Female mating frequencies from progeny of the two reciprocal F1 crosses were not significantly different, which suggest that the trait was autosomally inherited. Moreover, female mating frequency of F1 (pooled) progeny was not significantly different from the mid-parental value, which suggest no dominance. The computation of the Cavalli's joint scaling test consistently confirmed these results yielding values of d = 0.51 +/- 0.10 and h = 0.12 +/- 0.21. The broad sense heritability estimate was H2 = 0.73. It is concluded that polyandry in S. exigua is a polygenic, autosomal heritable trait and that additive genetic variance is available for selection for female mating frequency. The implications of the genetic basis of polyandry are briefly discussed in the context of current theories about this crucial insect mating system.