A preliminary analysis is presented of the viability effects of new mutations in heterozygous condition in three sorts of second-chromosome backgrounds: homozygous for chromosomes obtained from large populations, heterozygous for two chromosomes obtained from the same population, and heterozygous for two chromosomes obtained from widely separated populations of Drosophila melanogaster. The data are inconclusive, but in several types of analyses are consistent with the following suggestion: If a/sub i/ and a/sub j/ are two of many alleles at the a locus retained in a large population under the influence of natural selection, and if a/sub */ is newly induced allele, the order of viability of the different genotypes is a/sub i//a/sub j/ > a/sub i//a/ sub */ > a/sub i//a/sub i/ on the average. (auth)