The fungus Cercospora rosicola has been studied as a model system for abscisic acid biosynthesis. 1′-Deoxyabscisic acid and 4′-hydroxy-α-ionylidene acetic acid have been identified as endogenous compounds in this fungus. The results of feeding these and other putative intermediates suggest that abscisic acid biosynthesis proceeds via the successive oxidations of a 3 -methyl- 5 - (2′,6′,6′- trimethylcyclohex-2′-en-1′-yl)- 2,4-pentadienyl intermediate. Preliminary results suggest that a similar pathway may operate in plants.