Sixty inbred lines were established from a population cage and maintained for over 50 generations by sib mating in order to empirically determine the proportion that establish balanced lethal systems. Although in none of the 60 lines was a balanced lethal system detected, several lines contained segregating sterility factors. In addition, a study of the frequencies of the lethals found in the lines suggested that the lethal chromosomes were at least completely recessive. Moreover, tests indicated that these lethals, which were not restricted to a small number of loci, enhanced the viabilities of their heterozygous carriers. Finally, it was noted that the length of time an inbred line could be maintained was positively correlated with the frequency of lethals within that line.