The present study is concerned with experiment on radiation-mediated mutagenesis and with the analysis of the fine tructure of a gene locus in Drosophila hydei. It aimed at a more detailed knowledge of the genetic differences and correspondences between different species of the genus Drosophila. The scope of our actual comparisons is essentially limited to two species, viz. D. hydei and D. melanogaster, because of the scarcity of information available in other species. Certain extrapolations of a more general kind are nonetheless submitted, with the implicit understanding that pertinent experimental data are lacking to-date. Main conclusions are the following:

(1) -The genomes of the different species are invariant or barely different as to the number of genes they include.

(2) - Between species, the map length of the X chromosome varies widely, perfect similarity in genetic content notwithstanding. Essentially, the same holds true for the rest of the genome. This variation is not a function of physical length or DNA content of the chromosomes.

(3) - The intrinsic structure of homologous genes has not changed in the course of evolution of the Drosophila species, or at most has undergone minor modifications. In contrast, the arrangement of genes within the homologous chromosomes, or "Robertsonian" elements has been altered rather freely.

(4) - Experiments on D. hydei have shown that differences in karyotype, especially with respect to heterochromatin distribution and to acrocentric versus metacentric chromosome organization, may be decisive in either permitting or excluding the manifestation of particular mutation phenomena.