The galactose-1-phosphate uridyltransferase of the red alga Galdieria sulphuraria has been purified about 1800-fold to a final specific activity of approximately 140 U/mg protein. The purification involved chromatography on DEAE-Fractogel, hydroxyapatite, decyl-agarose, and DEAE-Tentacle gel. After SDS/PAGE, the enzyme preparation showed only one protein band of 42 kDa. The enzyme is a homodimer with a molecular mass of 82 kDa as estimated from the sedimentation velocity or 60 kDa as estimated by gel filtration. It has a broad pH optimum between pH 7 and pH 9. The apparent Km values for the forward and backward reactions are Km(Glc1P) = 105 μM, Km(UDP-galactose) = 30 μM, Km(Gal1P) = 400 μM, and Km(UDP-Glc) = 20 μM. The activation energy of the reaction is 45 kJ mol-1. The enzyme is specific for the galactose 1-phosphate to UDP-galactose interconversion in the Leloir pathway while the alternate enzyme for the Isselbacher pathway, UDP-galactose pyrophosphorylase, could not be detected in G. sulphuraria.