Isozyme variation in nine enzyme systems was assayed in five populations of E. spinosa representing all known occurrences of this species in Australia. All populations were monomorphic for all enzyme systems indicating each population was composed of a single, homozygous genotype. However, there were genetic differences among populations for three enzyme systems. Three of the five populations were distinguishable on the basis of the variable enzyme systems, the remaining two being identical. These results are discussed in relation to the history of introduction of E. spinosa into Australia and its potential for biological control.