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A regular pattern of morphogenetic responses to factorially arranged variations in concentrations of NH4Cl, K2HPO4 and glucose was found common to ten strains of Memnionella and Stachybotrys. Three phases of morphogenesis were observed: sterile swollen mycelium, sterile filamentous hyphae, and hyphae with conidiophores. The morphological status of the mature colony was found to be related both to the absolute concentrations of NH4Cl and K2HPO4, and to the ratio of these concentrations. At high values of the ratio the mycelium remained in the sterile swollen phase; at intermediate values the mycelium was mainly filamentous and sporing occurred; at low values the hyphae were filamentous but less fertile. Increase in concentration of K2HPO4 favoured filamentation whereas increase in the concentration of NH4Cl favoured swelling of the cells and suppressed sporulation. Glucose concentration determined mainly the extent of growth and intensity of sporulation, although some interaction between the effects of glucose and NH4Cl on sporulation were observed. A general set of conditions is proposed for the propagation of Memnionella and Stachybotrys in the sporing phase. It is suggested that the factorial design of nutritional experiment is appropriate for cultural studies fundamental to the taxonomy of fungi.