The effect of host cacti on the breeding structure of an isolated population of cactophilic Drosophila gouveai was studied. A comparison was made, using F statistics, of the allozyme frequencies at 11 loci among temporal samples of the adult population and six samples of progeny obtained from individual rotting cactus cladodes. The population appears to be structured by the cacti, forming breeding groups, and approximately four individuals contribute gametes to the progeny of each cladode. This D. gouveai population had a low degree of heterozygosity, compared with mean values for most Drosophila species, however it was within the range of values reported for other cactophilic Drosophila.