Blood serum samples (n=34) were taken from a population of vervet monkeys inhabiting thernterrain along the Awash River. Genetic variation on transferrin (Tf) and pseudocholinesterasern(Psch) were studied using a non-denaturing vertical polyacrylamide gel electrophoresisrnsystem. No variation was detected on transferrin. Psch was found to be polymorphic withrnrespect to mobility and activity level. Four phenotypes were distinguished based on thisrnpolymorphism. This included: single cathodal band with strong activity; single anodal bandrnwith strong activity; two bands of activity (one cathodal and the other anodal) and finally nornband activity. These phenotypes could be explained based on a three allele model proposedrnfrom a previous study. These alleles are designated as psch' for the silent gene, psch' for therncathodal band and psch' for anodal band activities. The genotypes of each individual werernconstructed and the gene frequencies calculated. The gene frequency for pSChl, psch' andrnpsch3 were 0.0883 and 0.3088 and 0.6029 respectively. Test for goodness-of-fit for thesernalleles showed the popUlation to be in a Hardy-Weinberg Equilibrium (X2rn3=4.95, p > .05)rnat present. However, when the results of this study were compared with a previous studyrn(some 20 years ago) on the same area, the allelic frequencies have greatly changed. Possiblernfactors responsible for the Tf monomorphism and Psch polymorphism are discussed. The Tfrnmonomorphism may be due to the low migration rate; the antagonistic effect of the Pschrnthird aile on rare Tf allele and the effect of sampling size. The shift in Psch allelicrnfrequency polymorphism may be accounted for by environmental factors. The effect of thern1974 and 1985 drought periods is may have contributed to the dominance of the third allele.rnThe Hitch-Hiking effect on the rapid shift of allele frequency and the effect of methods employed are also discussed. Based on the allelic distribution, important sites for thernconservation of vervet monkeys are recommended.