Mosquito cells from the C7-10 Aedes albopictus line were transfected with a recombinant plasmid containing the Escherichia coli galactokinase gene under control of the promoter from the Drosophila melanogaster metallothionein gene, Mtn. Consistent with what has been observed with heterologous metallothionein promoters in several vertebrate systems, treatment of transiently transfected mosquito cells with CuSO4 or CdCl2 induced a 2- to 5-fold increase in galactokinase gene expression. Levels of enzyme activity were not increased in tests using stably transformed lines despite wide ranges in the number of transfected gene copies detected in Southern blots. The importance of comparative studies with gene constructs that may eventually be used to produce genetically modified mosquitoes is underscored by the apparent variability in activity of heterologous promoters from D. melanogaster in different mosquito cell lines.