A defined medium for Cosmarium turpinii Bréb. was established. The optimal concentrations of major elements (Ca, N, P, S, and Mg) were detemiined, with the calcium level found to be most demanding. Iron, boron, and manganese were found to be essential micronutrient elements. Vitamin B(12) is also required, eitber as the planar group of cyanocobalamin alone or with adenine or benziniizole derivatives. Survival of cells during plateau pbase of growth can be extended by tbe addition of eitber 10(-3) M urea, isoleucine, or glycine.