SummaryExperiments with D. melanogaster were conducted to obtain information on the relative fitness of the structural heterozygote of a third chromosome inversion, In (3R)P, in five different genetic backgrounds.The results have shown that in all instances the inversion heterozygote was superior in fitness to both homozygotes and that the adaptive value of the chromosomal arrangement was conditioned by the genetic background.Estimates of 8 and t, the reduction in fitness of the standard and inversion homozygotes relative to the inversion heterozygote, were O· 25, 0'27, O· 33, O· 02, and 0·25,0·86,0·63,0·16, respectively, for each of the A, B, C, and D experimental populations, and § equalled 0·071 for the HL population.