Populations of L. cuprina from the Bairnsdale, Glenrowan, Hamilton and Torrita areas of Victoria were found to have similar high diazinon resistance status, following near fixation of a single allele at the R1 locus. Pure-breeding resistant strains were derived from each area and used as base populations for a selection program on adults. An approximate doubling of the level of resistance to diazinon was achieved in each strain after eight generations of selection. Relaxation of selection over seven generations showed the response had generated a stable plateau in all but the Torrita selected strain, which regressed towards the resistance level of the original base population. The genetic mechanism associated with the response to selection in each strain was autosomal but unique for each selected strain. Genetic effects directly influenced the resistance phenotype or acted through modification of the major locus. The possible reasons for the uniform-resistance status of the field populations are discussed in terms of the observed selection responses and the likely selection regimes operating in the laboratory and field.