Two-way selection for 21-day pupa weight was conducted in two highly inbred lines of Tribolium castaneum. The results, after 17 generations of selection, indicated that one of these lines (CSI-10) possessed a moderate amount of genetic variation for the trait selected (21-day pupa weight).When the selected populations were allowed to mate at random for 13 generations, the mean pupa weight regressed to values close to the means in the populations prior to selection. Reciprocal crosses between the high and low select lines revealed that 80% of the variation was associated with the sex chromosomes.The possibility that recurrent mutation was responsible for the genetic variation is discussed. It is concluded that natural selection favoring the heterozygous condition, rather than recurrent mutation, is responsible for the genetic variation. It is suggested that selection occurring between the sublines has reduced the rate at which the inbred line, CSI-10, is approaching complete homozygosity.