Sodium arsenite (0–4 mm) inhibited the growth of exponentially growing Bacillus cereus, whereas sodium arsenate was growth‐inhibitory only at 10 mm concentration. When the phosphate concentration was reduced sharply, 3.3 mm sodium arsenate inhibited growth, whereas the inhibitory effect of arsenite was independent of phosphate concentration. Neither arsenite nor arsenate produced any specific effects on the incorporation of precursors into ribonucleic and deoxyribonucleic acids, protein or cell wall, in support of the concept that their actions were at stages of energy utilisation rather than biosynthesis of macromolecules. Although some 74arsenate may have been bound by cells during inhibition of growth, 74arsenite was selectively and potently concentrated in micro‐organisms. There was no evidence for the formation of arsenic‐containing nucleic acids nor were labelled organic intermediates found. No instance of interconversion between pentavalent and trivalent states of arsenic could be established, and it was concluded that both compounds inhibited the growth of B. cereus by separate mechanisms.