The changes in plastid ultrastructure in the pericarp of cucumber (Cucumis sativus L.) fruit were studied during fruit yellowing (which accompanied maturation) and regreening. In the course of fruit maturation, the thylakoid system was progressively reduced, and only a small number of membranes remained in the plastids of mature fruit. At the same time, the plastoglobules increased in size, often remaining in close proximity to the degrading thylakoids. In pericarp tissue which turned green again, the thylakoid network in the plastids was gradually reconstituted. Morphological similarities between the plastids in mature and regreening fruit indicated that the chloroplasts in regreened tissue were redifferentiated from the plastids of mature fruit. Reconstitution of the thylakoid system appeared to start from two morphologically distinct types of membranes: from double membranes which resembled thylakoids and from membrane-bound bodies (MBBs). The latter appeared to form thylakoids by two mechanisms: by detachment of extensions from their surfaces and by fragmentation. The plastoglobules remained in the plastids during thylakoid system reconstitution and were often observed in close proximity to developing thylakoids. In the course of chloroplast redifferentiation, several types of membraneous structures were found to be associated with the plastid envelope: (i) vesicles which appeared to separate from the envelope and to fuse subsequently with the developing thylakoids, (ii) tubules, and (iii) double-membrane sheets which appeared as de novo forming thylakoids.