Fourteen enzymes of Ascaris lumbricoides were analysed electrophoretically on cellulose acetate plates; 8 stained well and 3 were found to be polymorphic. Allelic and genotypic frequencies at the 3 polymorphic enzyme loci, Mpi, Pgi and 6-Pgd, were determined in a sample of 117 worms from 8 children living in different houses in an urban slum in Bangladesh. Allele frequencies in samples of parasites from the different children were compared to test for the possibility of non-random distribution of parasite genotypes between people. No strong evidence of differences was found. Diploid genotype frequencies did not deviate significantly from those expected from the Hardy-Weinberg equilibrium. Significant linkage disequilibrium was observed in one of 3 pairs of enzyme loci tested, which might suggest that some genetic subdivision exists in the local A. lumbricoides population, although no strong interference should be made from this single result. Overall, the results suggest that the worms sampled formed part of a single population which appears to be randomly mating.