Bulk membrane fragments were prepared from cells of Bacillus cereus ATCC 4342 harvested at different stages of growth and sporulation and examined for enzymes involved in electron transport functions. The presence of succinate: DCPIP oxidoreductase (EC 184.108.40.206), succinate: cytochrome c oxidoreductase (EC 220.127.116.11), NADH:DCPIP oxidoreductase (EC 18.104.22.168), NADH:cytochrome c oxidoreductase (EC 22.214.171.124), succinate oxidase [succinate: (O2) oxidoreductase, EC 126.96.36.199], and NADH oxidase [NADH:(O2) oxidoreductase, EC 188.8.131.52] were demonstrated in membrane fragments from vegetative cells, early and late stationary-phase cells, and in cells undergoing sporulation. During the transition from a vegetative cell to a spore, there was a significant increase in the levels of enzymes associated with energy production via the electron transport system. Cytochromes of the a, b, and c type were detected in all membrane preparations; however, there was a marked increase in the level of cytochromes by the end of vegetative growth which remained throughout sporulation; there were no qualitative changes in the cytochromes throughout growth and sporulation. Sporulation was inhibited by cyanide, stressing the significance of the electron transport system. Enzyme activities were partially masked in washed membrane fragments; however, unmasking (stimulation) was achieved by sodium deoxycholate, sodium dodecyl sulfate, or Triton X-100. The degree of enzyme masking was less in vegetative cell membrane fragments than in membranes prepared from stationary-phase or sporulating cells. Results indicate the development of a membrane-bound electron transport system in B. cereus by the end of growth and prior to sporulation, which results in an increased masking of a number of enzymes associated with the terminal respiratory system of the cell.