Crude extracts of the following marine algae, cultured axenically under controlled conditions, were examined for l-threonine dehydratase activity: two cyanophytes ( Agmenellum quadruplicatum, Anacystis marina), one rhodophyte ( Porphyridium cruentum), two cryptophytes ( Chroomonas salina, Hemiselmis virescens), one chlorophyte ( Tetraselmis maculata), and one diatom ( Cyclotella nana). The specific activities (4–85 mμmoles/min/mg protein) showed considerable species differences as well as significant effects from the nutritional conditions of algal culture (autotrophic vs. heterotrophic). The enzymatic production of α-ketobutyrate from l-threonine was confirmed (with 2 species) by the isolation and chromatographic identification of its 2,4-dinitrophenylhydrazone. The algal enzymes showed pH optima in the range of 8·5–9·5 and 'sigmoid' or 'paraboloid' kinetic response to threonine concentration. With the exception of C. nana, all the algal enzymes were strongly inhibited by l-isoleucine; l-Valine, EDTA, AMP, ADP, and cyclic 3′,5′-AMP had no significant effect. Several carbonyl reagents (hydrazine, hydroxylamine, etc.) strongly inhibited enzyme activity and this inhibition was reversed to varying degrees by pyridoxal 5′-phosphate. Excepting iodoacetamide, all the reagents known to modify protein sulfhydryl groups inhibited the activity strongly and these inhibitions were partially reversed by dithiothreitol. The enzymes of C. salina and H. virescens were also strongly inhibited by dithiothreitol. These results characterized the algal enzymes generally as isoleucine-regulated, pyridoxal-phosphate requiring, allosteric threonine dehydratases similar to the corresponding 'biosynthetic' enzymes previously reported for bacteria, fungi, and higher plants. In addition, the algal enzymes appeared to require sulfhydryl groups for the expression of activity. The C. nana enzyme appeared to be insensitive to isoleucine regulation, and the cryptophyte enzymes appeared to require disulfide groups for activity.