A pure culture of Phlyctochytrium reinboldtae Persiel isolated from soil near Ottawa, Canada, grew best at 20 and 25 C, slightly at 10 C, and not at all at 35 C. It grew best in alkaline media and required no exogenous vitamins. It used ammonium, nitrate, and nitrite nitrogen and each of the 7 amino acids tested. Of 18 carbon sources, it used glucose, fructose, trehalose, maltose, mannose, cellobiose, glycerol, and starch. It is similar physiologically to P. punctatum Koch and P. plurigibbosum Barr and morphologically to P. papillatum Sparrow and P. dichotomum Umphlett.