Two varieties of Rhizophydium sphaerocarpum (Zopf) Fischer are proposed: var. sphaerocarpum, saprophytic on algae, nematodes, and pine pollen; and var. spirogyrae, a parasite of Spirogyra spp. The varieties differ morphologically only in colony color and texture. Variety sphaerocarpum grew best at (10–) 15–20 C and used inorganic nitrogen as ammonium chloride, ammonium nitrate, and sodium nitrate when the inoculum contained actively growing thalli, whereas var. spirogyrae grew best at (25–) 30 C and did not use inorganic nitrogen. Results of pH range tests and carbon nutrition were very similar for the two varieties and both required exogenous thiamine.