To test the suitability of DNA stable isotope probing (DNA-SIP) for characterizing bacterial spore populations in soils, the properties of Bacillus subtilis cells and spores intensely labeled with [C-13]glucose were characterized. Spore germination, vegetative growth rates, and sporulation efficiency were indistinguishable on glucose versus [C-13]glucose, as were spore wet heat and UV resistance. Unlabeled and C-13-labeled spores contained 1.0989 and 74.336 at.% C-13, and exhibited wet densities of 1.356 and 1.365 g/ml, respectively. Chromosomal DNAs containing C-12 versus C-13 were readily separated by their different buoyant densities in cesium chloride/ethidium bromide gradients.