Adenovirus class B (Ad3 and Ad7) and class C (Ad1, Ad5, and Ad6) late r-strand mRNA's were found to have segmented 5' leaders. These leaders were very similar among serotypes within a class but differed in sequence from the leaders on late mRNA's of a different class. However, the leader components of class B viruses mapped at essentially the same map coordinates as those of class C viruses. The 5' coordinates of the main bodies of class B messages of which the tripartite leaders are attached as well as the map positions of several of their early mRNA's were very similar to those of Ad2 transcripts. Infrequent examples of late r-strand polysomal RNAs of Ad3 and Ad7 had, in addition to the three common leader segments, a fourth leader segment derived from RNA encoded at various sites between the second and third leaders. The extra components formed several distinct groups. These molecules are presumably intermediates in the splicing processes that generate mature messages.