Retroviral genomes with a high frequency of G-to-A mutations are thought to originate during reverse transcription. Here we show that bursts of G-to-A mutation may also occur during DNA synthesis by Taq polymerase on a simian immunodeficiency proviral template. These G-to-A changes tend to occur at GpA and, to a lesser extent, GpG dinucleotides. Because the resulting sequences are like previously reported hypermutant human and simian immunodeficiency virus (HIV and SIV) genomes, it is important to design experiments that can clearly discriminate between Taq and reverse transcripts errors in studies of lentiviral G-to-A hypermutation.