Gene polymorphisms involved in the metabolism of drugs and chemical carcinogens seem to be responsible for differences in the susceptibility of individuals to cancer, but genetic population studies are needed to characterize these polymorphisms in different ethnic populations. We investigated polymorphisms of the cytochrome P450 (CYP) gene CYP1A1 and the glutathione S-transferase (GSTs) genes GSTM1, GSTT1 and GSTP1 in a sample of Afro-Brazilians from the southern Brazilian city of Porto Alegre to verify if there were ethnic differences compared to the polymorphisms of the same genes in a previously described sample of Brazilians of European descent from the same city. The allele frequencies detected in the Afro-Brazilian population investigated in this study were CYP1A1*2A (30%) and GSTP1*Val (42%) while the frequency of the GSTM1 null genotype was 34% and that of the GSTT1 null genotype was 28%. Significant differences in genotype distribution and allelic frequencies were detected between Brazilians of African and of European descent from Porto Alegre in terms of the polymorphisms CYP1A1*2A (p = 0.003), GSTP1-Ile105Val (p = 0.002) and the GSTM1 null genotype (p = 0.01) but there was no detectable significant difference in respect to GSTT1 null genotype frequencies.