Three size classes of human immunodeficiency virus type 1 (HIV-1) mRNAs are produced in infected cells: full-length, intermediate, and small. Here we report that the intermediate-size class of viral mRNAs is heterogeneous, consisting of at least 12 differentially spliced species. This group contains nine bicistronic mRNAs producing Env and Vpu and three mRNAs expressing only the first exon of tat. In the latter mRNAs, Env and Vpu expression is blocked by the presence of the upstream tat open reading frame. We conclude that internal initiation of translation is not the mechanism for generation of the bicistronic env mRNAs. Translation of HIV-1 mRNAs is consistent with the scanning mechanism in which Env is produced by leaky scanning from mRNAs that contain env as the second or third reading frame. Env and Vpu proteins are expressed from the same mRNAs and are coordinately regulated by Rev. This arrangement may reflect a requirement for coordinate expression of Vpu and Env.