Over the last two decades, significant data has been accumulated linking Glutatione S-Transferases (GSTs) with the development of several diseases. Contemporary studies have demonstrated the impact of ethnicity on GST allele frequencies. The aim is to verify if the variability of GST genes reflects population demographic history or rather selective pressures. GST genes (GSTM1, GSTO1 GSTO2, GSTT1) were analysed in three Ecuadorian populations (Cayapas, n=114; Colorados, n=104; African-Ecuadorian, n=77) and compared with HapMap data. GST SNPs were determined using the PCR-RFLP method while GST null phenotype was determined using a Multiplex PCR. The population relationship achieved using GSTM1 positive/null, GSTO1*A140D, GSTO2*N142D and GSTT1 positive/null are in agreement with the data obtained using neutral polymorphisms: Amerindians are close to Asian populations and African-Ecuadorians to African populations. To what concerns GSTO1*del155 and GSTO1*K208 variants, allele frequencies never exceeded 10%, showing no significant differences in the Ecuadorian groups and in worldwide populations. The features of GSTO1*del155 and GSTO1*K208 variants and their association with arsenic biotransformation deficiency suggest the presence of a selection mechanism towards these loci. In particular, this hypothesis is strengthened by a possible linkage between these alleles and the susceptibility of arsenic-induced male infertility.