The gag p6 region of HIV-1 has various nonsubstitutionary mutations, including insertions, duplications, deletions, and premature stop codons. Studies have linked gag p6 mutations to reduced susceptibility to antiretroviral therapy in HIV-1 subtype B. This study examined the relationship between antiretroviral therapy and gag p6 diversity in HIV-1 CRF02_AG and subtype G. p6 data were generated for secondary analyses following Viroseq genotyping of pol gene sequences in plasma samples from HIV-1-infected Nigerians on reverse transcriptase inhibitor therapy, with virologic failure (repeat VL > 2000 copies/ml). p6 sequence chromatograms were available for 40 CRF02_AG and 43 subtype G-infected individuals. Subjects who had not received their supply of antiretroviral drugs for at least 2 months prior to the plasma sampling were classified as nonadherent. p6 sequences from therapy-adherent individuals had more nonsubstitutionary mutations than sequences from drug-naive individuals (p = 0.0005). The P5L/T mutation was inversely correlated with the presence of K27Q/N in p6, with each mutation being more prominent in subtype G and CRF02_AG, respectively. The data also suggested that P5L/T may be a compensatory mutation for the loss of an essential phosphorylation site in p6. In addition, there was an inverse association between P5L/T mutations in p6 and thymidine analog mutations in reverse transcriptase (p = 0.0001), and drug nonadherence was associated with an 8-fold lower risk of having a nonsubstitutionary mutation in p6 (95% CI = 1.27-52.57). Our data suggest that antiretroviral therapy influences gag p6 diversity, but further studies are needed to clarify these observations. © 2008 Mary Ann Liebert, Inc.