Three strains of Salmonella typhimurium carrying frameshift mutations affectin g the histidine genes (hisC3076, hisD3052 and hisC207) showed increased sensitivity to mutagenesis by ICR-191 (as judged by measuring back mutation to prototrophy), if they were made deficient in excision repair by deleting the uvrB gene. One frameshift strain, hisC3076, also showed increased sensitivity to mutagenesis by ICR-191 when it carried either of two different polA alleles, whereas the hisD3052 and hisC207 frameshifts reduced sensitivity to mutagenesis in the presence of these alleles. Studies of spontaneous back mutation to prototrophy revealed significant mutator effects of the polA1 mutation on reversion of the hisD3052 frameshift and of the polA3 mutation on reversion of the hisC3076 frameshift. Other smaller mutator effects of the polA alleles on complex interactions between different polA alleles and different frameshift mutations, it is tentatively suggested that deletion frameshifts may arise mainly during DNA replication, while addition frameshifts may arise mainly during post-replication repair.