Low activity (about 4 mU/mg protein) of 5'-deoxyadenosylcobalamin-dependent methylmalonyl-CoA mutase (MCM; EC 18.104.22.168) was found in a cell homogenate of a photosynthetic coccolithophorid alga, Pleurochrysis carterae. Most of the enzyme occurred as the apo-enzyme, which was labile during purification. The holo-enzyme, which was converted from the apo-enzyme by incubation with 10 microM 5'-deoxyadenosylcobalamin at 4 degrees C in the dark, was purified to homogeneity and partially characterized. An apparent molecular mass for the enzyme of 150+/-5 kDa was calculated by Superdex 200 pg gel filtration. SDS-polyacrylamide gel electrophoresis of the purified enzyme gave a single protein band with an apparent molecular mass of 80+/-5 kDa, indicating that the P. carterae enzyme occurs as a homodimer. Some properties of methylmalonyl-CoA mutase from P. carterae were studied.