Cell extracts of the C115 (β-carotene-accumulating) strain of Phycomyces blakesleeanus were incubated with either [2-14C]MVA or [1-14C]IPP and a range of possible inhibitors of carotenogenesis, including bleaching herbicides, biphenyl compounds and geranylacetone. Several of these compounds were potent inhibitors of β-carotene formation and caused the accumulation of phytoene. No other carotenes were found to accumulate, in vitro.The structures of these inhibitors, compared to that of phytoene, suggest that they affect the enzymic activity of “phytoene dehydrogenase”, possibly by competitive inhibition.