The human foamy or spumaretrovirus HFV is a complex and exogenous retrovirus that encodes several bel genes besides the three classical retroviral genes gag, pol, and env. HFV was recently reported to contain two functionally active promoters that are both strongly trans-activated by the HFV trans-activator protein Bel 1. The occurrence of a second internal cap site underscores the complexity of the HFV genome. We have analysed whether there is interference between the HFV long terminal repeat promoter and the internal promoter located in the 3' end of env upstream of the bel genes. Recombinant clones were constructed that carry two different indicator genes, one under the control of the U3 promoter, the other under the control of the internal promoter. The portion of the basal transcriptional activity of the internal promoter that is not trans-activated by Bel 1 was increased two- to threefold in the presence of the long terminal repeat promoter. The rate of trans-activation by Bel 1 of both HFV promoters was not altered in these constructs.