Several infections ofRhynchosciara angelaeby Rhynchosciara polyhedrosis virus (RPV) were studied. The virus multiplies within the nuclei of the cells of the intestinal caeca and the mid-intestine. The viruses cause the formation of unicellular tumors with greatly enlarged polytene chromosomes which may have constrictions at specific points. The nucleoplasm of the nuclei with enlarged chromosomes may be Feulgen-positive or -negative. Autoradiographic studies of nuclei of larvae injected with H3-thymidine indicate that the multiplication of the virus may be connected with the host chromosomal material. Four different constrictions may be induced by the virus in one of the enlarged chromosomes (A). These constrictions are located in the neighborhood of four thick bands, two of which are typically heterochromatic and the other two which, although not having the « vacuolized » aspect of heterochromatin, sometimes may ectopically pair with heterochromatic regions of the chromosomes. Several different strains of virus were studied; all of them induce constrictions near the two heterochromatic bands. Each one of the other two constrictions may be present or absent depending on the strain of the virus. Therefore, the specificity of the constriction locations depends not only on the structure of the chromosome (distribution of heterochromatic regions), but also on the strain of the virus. In some of the infected cells the chromosomes show an intense activity in RNA production with an increase in the total volume of the chromosome and the production of a great number of micronucleoli. Other infected cells have chromosomes which are greatly increased in length but only slightly increased in width. The implications of these observations are discussed in relation to the effect of virus infections on chromosomes of mitotic cells.