The growth of C. obscurus was studied in solid and liquid media. In agar media, the hyphal growth was linear, but the number of apices increased exponentially; total growth was consequently exponential but characterized by a low growth rate. In batch cultures in a fermenter, mycelial growth was continuously linear following a short exponential phase. The absence of branching induced a linear development of the fungus in liquid culture. Fermentation balances showed that the linear growth could not result from the presence of a specific inhibitor, nor from the existence of a limiting concentration of a nutrient in the medium. Hypotheses concerning the non-branching of the fungus in liquid culture are discussed.