Trypanosoma brucei stocks (43) isolated from man, cattle and tsetse, all coming from a small area of Lambwe Valley, in Kenya were characterized using isoenzyme analysis. The tsetse isolates revealed the largest number of zymodemes, indicating a lot of heterogeneity among these isolates. Cluster analysis of all the trypanosomes characterized showed that the organisms could be grouped into three distinct trypanosome types; human, cattle and tsetse. The human type was represented in ail the three groups. This was not the case either with the cattle nor the tsetse T. brucei type. T. brucei zymodeme which was found to be predominant during the 1980 outbreak of sleeping sickness was not observed this time round. It is suggested that this zymodeme was either unstable and had changed or it had been eliminated during the insecticides ground spraying operations against G. pallidipes. It is concluded that the heterogeneity observed among the fly isolates is an adaptive mechanism for survival of trypanosomes.
