The formation of endospores in bacteria is generally studied in media that do not support growth, such as exhausted growth medium or mineral salt solutions. Under carefully controlled conditions, a reasonably synchronous sporulation can be achieved in most populations. There is no need or possibility for changing conventional methods to study sporulation. In 1961, it was observed that spores are constantly formed in cultures of Bacillus megaterium growing exponentially in a mineral-glucose medium. Similar results have been obtained with B. subtilis. Relative to the total viable population, the number of spores first reaches a limit and then increases at a constant rate, equal to the growth rate of the total population. This study investigates the influence of various factors on the size of the sporulated fraction (ρ) in growing cultures of B. subtilis. The interpretation of the results suggests that the probability for a cell in a growth medium to become committed to sporulate must be determined by the intracellular concentration of at least one nitrogen-containing catabolite repressing the expression of all sporulation genes. The study used the wild Marburg strain of B. subtilis. A common mineral base was prepared from three solutions. Various substances were added as carbon or nitrogen sources. A complex medium was also used. Stock cultures were kept in NH4Cl-glucose medium at -40°C in the presence of glycerol. Precultures were made in the selected medium, and cultures were shaken at 37°C. Turbidity measurements, viable counts, and spore counts were made during exponential growth. Spore counts were made after heating a sample for 10 minutes at 80°C. Theoretical considerations and presentation of results: During exponential growth, the total bacterial population (NT) consists of three kinds of cells: growing cells (NG), cells committed to sporulate (NC), and thermoresistant spores (NS). The growth rate (μ), sporulated fraction (ρ), probability for a cell to become committed to sporulate (π), and time required for a committed cell to reach thermoresistance (τ) were analyzed. The values of these parameters were derived from experimental data. Results: The influence of the nature of the carbon or nitrogen source was studied. The probability for a cell to become committed to sporulate (π) was found to depend on the composition of the medium. With NH4Cl as the nitrogen source, π was of the order of 10 percent with most carbon sources, but much smaller when malate was the substrate. In the presence of glucose, π varied within wide limits, being negligible when all amino acids were supplied and approaching unity when histidine was the only nitrogen source. The influence of the concentration of the carbon and energy source was also studied. The effect of varying the concentration of the carbon source was presented in Table 2. In the range of concentration studied, μ was concentration-independentThe formation of endospores in bacteria is generally studied in media that do not support growth, such as exhausted growth medium or mineral salt solutions. Under carefully controlled conditions, a reasonably synchronous sporulation can be achieved in most populations. There is no need or possibility for changing conventional methods to study sporulation. In 1961, it was observed that spores are constantly formed in cultures of Bacillus megaterium growing exponentially in a mineral-glucose medium. Similar results have been obtained with B. subtilis. Relative to the total viable population, the number of spores first reaches a limit and then increases at a constant rate, equal to the growth rate of the total population. This study investigates the influence of various factors on the size of the sporulated fraction (ρ) in growing cultures of B. subtilis. The interpretation of the results suggests that the probability for a cell in a growth medium to become committed to sporulate must be determined by the intracellular concentration of at least one nitrogen-containing catabolite repressing the expression of all sporulation genes. The study used the wild Marburg strain of B. subtilis. A common mineral base was prepared from three solutions. Various substances were added as carbon or nitrogen sources. A complex medium was also used. Stock cultures were kept in NH4Cl-glucose medium at -40°C in the presence of glycerol. Precultures were made in the selected medium, and cultures were shaken at 37°C. Turbidity measurements, viable counts, and spore counts were made during exponential growth. Spore counts were made after heating a sample for 10 minutes at 80°C. Theoretical considerations and presentation of results: During exponential growth, the total bacterial population (NT) consists of three kinds of cells: growing cells (NG), cells committed to sporulate (NC), and thermoresistant spores (NS). The growth rate (μ), sporulated fraction (ρ), probability for a cell to become committed to sporulate (π), and time required for a committed cell to reach thermoresistance (τ) were analyzed. The values of these parameters were derived from experimental data. Results: The influence of the nature of the carbon or nitrogen source was studied. The probability for a cell to become committed to sporulate (π) was found to depend on the composition of the medium. With NH4Cl as the nitrogen source, π was of the order of 10 percent with most carbon sources, but much smaller when malate was the substrate. In the presence of glucose, π varied within wide limits, being negligible when all amino acids were supplied and approaching unity when histidine was the only nitrogen source. The influence of the concentration of the carbon and energy source was also studied. The effect of varying the concentration of the carbon source was presented in Table 2. In the range of concentration studied, μ was concentration-independent