WHAT are the interrelationships of nonhomologous chromosomes in crossing over? This problem, important in a consideration of the basic activity of the nucleus as a whole, has sporadically occupied the attention of some Drosophila workers for many years. The most striking of the interchromosomal effects have come from mothers containing chromosome aberrations. For example, typical effects depend upon the presence of given heterozygous inversions in a chromosome pair or pairs, resulting in marked changes in recombination frequency in still another pair. These changes are largely increases in recombination and are influenced by regional position along the chromosome measured; they are rather easily followed, and are well established. But in addition to these special effects depending upon gross structural heterozygosity, there have been further indications of the existence in structurally standard females, of interchromosomal correlations of crossing over within individual nuclei. These latter associations appear to be relatively small, and they are not very easily investigated. What evidence there is suggests that they vary in value, and possibly even in sign, depending upon the specific regions involved. Earlier statements concerning results of SCHULTZ and REDFIELD and of SCHULTZ and BRIDGES may be found in the Carnegie reports (MORGAN, BRIDGES, and SCHULTZ 1930, 1931) ; these and related results to 1951 have been reviewed by SCHULTZ and REDFIELD (1951 ) . But the situation is not clear, and the conclusions are tentative. Definitive data are needed for the simple standard female. We must know more about the generalized interchromosomal relationships in the structural homozygote because these are basic to the problem as a whole, and because the general effects are not necessarily dependent upon precisely the same set of causal factors as those resulting in the marked effects found in the aberrant types. Thus a re-evaluation of the general situation in the structural homozygote now seems desirable, primarily because of its basic significance in biological termsbut also in the hope of a clearer understanding of the modifications taking place in the specialized types. This was the double purpose of the present experiments. This work was supported by a grant (C-1613) to DR. JACK SCHULTZ from the National Cancer Institute of the National Institutes of Health, Public Health Service, and by an institutional grant of the American Cancer Society.