Phylogenetic analysis of partial env sequences of HIV-1 isolates from Cyprus and Greece suggested the existence of a distinct subtype of the virus, designated as I. We examined whether this subtype represents a distinct group, or a mosaic consisting of previously characterized subtypes. The full-length sequences under consideration were recovered from serum samples of "subtype I" obtained from two nonepidemiologically linked HIV-1-infected subjects in Greece. The first subject was an intravenous drug user (IDU), while the second was a vertically infected child born in 1984 whose parents were both IDUs. A variety of methods, such as diversity plots as well as phylogenetic and informative site analyses, were used to classify the DNA sequences. Subsequent detailed analysis revealed a unique genomic organization composed of alternating portions of subtypes A, G, and I. The two Greek isolates formed a distinct group in most of the pol, gp120, and gp41 regions, and in the vif/vpr, vpu, LTR, and 5' terminus of nef. In contrast, different parts of env and gag as well as the 3' pol region, and the first exons of tat and rev, appeared to have arisen from subtypes A and G. Our results indicate that subtype I, which was probably circulating in Greece in the early 1980s, is a triple mosaic consisting of A, G, and I sequences.