1. The incorporation of (3RS)-[2−14C,(4R)-4−3H1]mevalonic acid into various cyclic carotenes in the fruit of the tomato mutant delta has been studied. The results confirm our previous view that the α-ionone ring of α-carotene does not arise by isomerization of a β-ionone residue, and show that the same is also true for the α-ionone ring of δ- and ∈-carotene and α-zeacarotene. 2. The incorporation of (3RS)-[2−14C,2−3H2]mevalonic acid into α- and β-carotene in carrot roots has been studied. The results show that the β-ionone ring of β-carotene does not arise by isomerization of the α-ionone residue of α-carotene. 3. These experiments show that α- and β-ionone rings in cyclic carotenes are formed independently, probably by elimination of different protons from the same carbonium ion intermediates.