The gene encoding the 51-kDa subunit of nicotinamide adenine dinucleotide (NADH) dehydrogenase complex I, a principal component of the mitochondrial electron transport chain, was cloned in Candida tropicalis. The homolog in C. albicans, CaNDH51, was identified, and each allele was successively disrupted by PCR-mediated gene disruption. Wild type, heterozygote, reintegrant, and homozygous null mutants grew as blastoconidia in rich medium containing 3% glucose, but the homozygous null mutant failed to grow in ethanol or acetate. When glucose concentration was varied from 1 mM (0.018%) to 200 mM (3.6%) in a basal salts medium, all strains grew equally well at all glucose concentrations; the wild-type strain, the heterozygote, and the reintegrant exhibited abundant germ tubes, pseudohyphae, and hyphae. In contrast, the ndh51/ndh51 strain failed to display any type of filamentous growth, even in glucose concentrations as low as 1 mM. These results suggest a previously unexplored relationship between mitochondrial electron transport and morphogenesis.