Cytosolic NADP‐dependent methylenetetrahydrofolate dehydrogenase‐cyclohydrolase synthetase and the mitochondrial NAD‐dependent methylenetetrahydrofolate dehydrogenase‐cyclohydrolase (NMDMC) are differentially expressed during insect development although both enzymes are detectable at all stages. In contrast, cell lines derived from a variety of insect species express high levels of NMDMC but undetectable levels of the NADP‐dependent enzyme. Northern analysis indicates the NMDMC message is expressed at levels 50–100 times higher in a Drosophila cell line compared to adult flies. RNase protection showed the predominance of shortened transcripts that require initiation at a downstream AUG producing a truncated protein that lacks a mitochondrial targeting sequence. These changes in expression effectively exchange the cytosolic NADP‐dependent dehydrogenase for one with NAD specificity.