During dark incubation of excised leaf tissue from Bf 993, a senescence mutant of Festuca pratensis Huds., a highly polar green pigment fraction (PC) increased from negligible amounts at day 0 to maximal concentrations at days 4–6 and declined thereafter. Over the same time-scale, chlorophyll (chl) was lost from leaves of the normal genotype Rossa without any detectable accumulation of PC. Chromatographic analysis established that PCs are a group of dephytylated derivatives of chl a. Chl- protein complexes from 4–day-senesced Bf 993 tissue were isolated by sucrose gradient centrifugation and PCs identified by thin-layer chromatography. High levels of PC were associated with the light-harvesting complex of Photosystem II; Photosystem I yielded none. Cycloheximide, at a concentration sufficient to inhibit yellowing in the normal genotype, completely abolished the appearance of PC in Bf 993. Accumulation of PC-like pigments in Rossa leaf tissue could be induced by anoxia. It is proposed that PCs are normal intermediates in chl catabolism, which begins with a protein synthesis-dependent dephytylation, followed by oxidation leading ultimately to open-chain pyrrole derivatives. The first steps in the breakdown sequence take place while the pigment is associated with proteins in the thylakoid membrane. The metabolic lesion in the Festuca mutant appears to be located downstream of the dephytylation step, leading to an abnormal accumulation of PCs in this genotype.