Wild-caught Xenopsylla brasiliensis from Kibwawa village, Chunya District, in south-west Tanzania were colonised in the laboratory and their offsprings used for determining the baseline susceptibility levels of the strain to DDT. A total of 120 fleas were tested with each concentration of DDT, using the techniques described by the WHO Expert Committee on Insecticides (1970).

Several filial generations were selectively raised, using DDT as a selection pressure. They were tested with various concentrations of the insecticide and their dosage–mortality regression lines compared with that of their parents.

About 5% of the parental generation (normal population) survived 1 hr exposure to 4% DDT. The LC50 of this population was 0.79% DDT. The mortality rates of the successive filial generations decreased gradually; their dosage–mortality regression lines moved to the right; and the slopes of these lines decreased. LC50s of F1, F2 and F3 were 1.6, 1.9 and 3.1% DDT, respectively. The highest DDT concentration (4%) killed 21.1 and 18.3%, of F4 and F5, respectively.

It was generally concluded that DDT resistance was present in the population, and that the selection pressure (DDT) was responsible for increasing the level of this resistance in the laboratory.