Lactate dehydrogenases [D(-)-lactate: NAD oxidoreductase, E.C. 1.1.1.28] have recently been detected in a number of lower fungi particularly among the Oomycetes (Gleason and Price, 1969; Warren and Mullins, 1969; LeJohn, 1971) and the Chytridiomycetes (Gleason and Price, 1969; LeJohn, 1971). The Oomycetes known to synthesize these enzymes are Apodachlya brachynema (Hildebrand) Pringsheim, Araiospora sp., Sapromyces elongatus (Cornu) Coker, Mindeniella spinospora Kanouse, Rhipidium sp., Aqualinderella fermentans Emerson & Weston, Achlya ambisexualis Raper, Aphanomyces laevis DeBary, Pythium ultimum Trow, and Phytophthora dreschleri Tucker (Gleason and Price, 1969; Warren and Mullins, 1969; LeJohn, 1971; Gleason, unpublished data). The purpose of this study was to compare certain physical and kinetic properties of the lactate dehydrogenases from several of the Oomycetes listed above. The fungi used for this investigation were kindly made available by the following: Pythium ultimum (67-1) by Dr. Joseph Hancock, Department of Plant Pathology, University of California, Berkeley; Aphanomyces laevis (107-52) by Dr. Torgny Unestam, Institute of Physiological Botany, University of Uppsala; and the members of the Leptomitales by Dr. Ralph Emerson, Department of Botany, University of California, Berkeley. The methods for growth of the fungi and for preparation of cellfree extracts were the same as those described by Gleason and Price (1969). The reaction rates were measured spectrophotometrically at room temperature by a Zeiss PMQII spectrophotometer equipped with a strip chart recorder. The standard assay mixture for the backward reaction contained sodium pyruvate (3.3 X 10-4 M), reduced nicotinamide-adenine dinucleotide (NADH) (1.4 x 10-4 M), potassium phosphate buffer (0.1 M, pH 7.0), and enzyme in a final volume of 3.0 ml. The standard assay mixture for the forward reaction contained D(-)lithium lactate (0.1 M), NAD (2.0 X 10-4 M), tris hydrochloride buffer (0.1 M, pH 8.5), and enzyme in a final volume of 3.0 ml. The rate of the reaction was determined by change in absorbance at 340 mu