Fructose 1,6-diphosphate aldolase from cells of Bacillus cereus appears to be typical Class II aldolase as judged by its functional and physical properties. Spore and vegetative cell aldolase had similar enzymatic, immunochemical, and heat resistance properties in the absence of calcium, but they differed in their thermal stabilities in the presence of calcium, their Stokes' radii, their mobility in acrylamide gel electrophoresis, and their molecular weights. The pH optimum for both enzymes was 8.5, and their K(m) with respect to substrate was 2 x 10(-3)m. Highly purified spore and vegetative cell aldolases were both heat labile with half-lives of 4 min at 53 C and pH 6.4. In the presence of 3 x 10(-2)m solution of calcium ions, the stability of the spore protein increased 12-fold but the vegetative form became more heat labile. The enhanced stability of the spore aldolase was not diminished by dialysis or gel filtration but was lost after chromatography on diethylaminoethyl cellulose at pH 7.4. Aldolase from vegetative cells exists in an equilibrium mixture of two molecular weights, 115,000 and 79,000 in the approximate ratio of 1:4, respectively. The molecular weight of spore aldolase is 44,000. Spore aldolase was more mobile during electrophoresis than its vegetative cell counterpart because of its smaller size.