Apparent selection affecting starch gel electrophoretic alleles at the 'Esterase-2' locus of 'Drosophila buzzatii' has been detected in laboratory and natural populations. Perturbation-reperturbation of allele frequencies in replicated laboratory populations attempts to test direct selective effects at the locus versus effects of linked loci. Sequential gel electrophoresis has identified more alleles within starch classes, and three of these alleles (within the a, b and c starch alleles) were used in cage population experiments. Allele 'a/1.00/1.00/1.00' was set up in 10 replicate populations with allele 'c/1.00/1.00/1.00', and in an independent 10 replicate populations with allele 'b/0.99/1.01/1.00'. For each set, three reperturbations were done. Replicate populations generally showed similar patterns of allele frequency change and clear directionality: effects of selection, not drift. However, four populations deviated from their replicates, indicating issipation of linkage disequilibrium. Estimates of pre-adult viability in the F2 of pair-wise crosses among 12 sequential gel electrophoretic alleles showed very variable modes of inheritance and relative viability fitnesses. Together with the diversity of patterns of allele frequency change in the cage populations, these results suggest a gene complex, with selection acting on an interacting set of loci which may include 'Esterase'-2.