The phenotypes of several heterozygous combinations of mutations which map within the Ultrabithorax gene of Drosophila melanogaster are modulated by the extent of somatic homologous chromosome pairing, an effect known as transvection. One can discriminate between otherwise phenotypically similar mutations via their transvection behavior. This suggested the existence of previously undetected intragenic functional units. A collection of mutations has been classified into "transvection groups" (in analogy to complementation groups) on the basis of transvection tests with bithorax34e, postbithorax2, and Contrabithorax1 Ultrabithorax1. The conditions necessary for each transvection effect were determined from these transvection groups. The bithorax34e mutation only transvects with Ultrabithorax mutations with a contiguous Ultrabithorax transcriptional unit. In contrast, postbithorax2 transvection requires the distal part of the bithoraxoid region. As expected, Ultrabithorax mutations do not transvect with Contrabithorax1 Ultrabithorax1. However, it appears that this cross activation is not mediated solely through one of the known regulatory regions as mutations in these regions do not consistently block the response.