Succinate dehydrogenase (SDH) was isolated from several fungi and the properties of the Mucor rouxii and Penicillium duponti SDH's were examined in some detail. The level of SDH was found to be dependent on the carbon source used for growth. The SDH of M. rouxii was found in the soluble fraction whereas the SDH of P. duponti was particulate. There is no evidence of more than one form of SDH in either M. rouxii or P. duponti. The SDH's from M. rouxii and P. duponti have different migration patterns after electrophoresis on polyacrylamide gels and have molecular weights of 110 000 ± 10 000 and 160 000 ± 10 000, respectively. M. rouxii SDH had a Km succinate of 3.7 × 10−4 M, a Ki malonate of 27.0 μM, and a Ki fumarate of 1.5 mM. P. duponti SDH had a Km succinate of 4.6 × 10−4 M, a Ki malonate of 17.5 μM, and a Ki fumarate of 2.6 mM.