Cell-free extracts of mycelia and sclerotia ofSclerotinia sclerotiorum (Lib.) D By. grown on synthetic liquid medium with various carbon sources containedd-mannitol-1-phosphate: NAD oxidoreductase (EC 1.1.1.17) and weakd-mannitol: NADP oxidoreductase (EC 1.1.1.67) activity. The difference in specific activity of the two enzymes coupled with the presence ofd-mannitol-1-phosphatase activity indicates the major pathway for mannitol synthesis. $$\begin{gathered} D - Fructose - 6 - P + NADH + H^ + \mathop \rightleftharpoons \limits_2^1 D - Mannitol - 1 - P + NAD^ + \hfill \\ D - Mannitol - 1 - P\xrightarrow{{ 3 }}D - Mannitol + P_i \hfill \\ \end{gathered} $$ . Both enzymes were present in mycelia and sclerotia at all times, but highest activity occurred during early stages of growth. The optimum pH for reactions 1,2 and 3 was 7.5, 10.5 and 6.5 respectively. The oxidoreductase was quite specific in substrate requirements. Both enzymes were inhibited by sulfhydryl reagents and the phosphatase was activated by Mg2+.