Purpose. The goal of these studies was to identify defining characteristics and critical periods in the embryonic development of vertebrate retinal Müller glial cells to facilitate a targeted attack on the regulation of Müller cell differentiation. Methods. Immunohistochemical methods were used to establish the developmental appearance of Müller cell markers and morphologically distinct trans-retinal radial cells relative to:1mitotic activity;2appearance of neuronal markers;3age;4retinal position. Müller cell markers used included:glutamine synthetase;carbonic anhydrase-II;2M6 antigen. Neuronal markers included:opsin;N-CAM;HuD RNA-binding protein;HNK-1 carbohydrate;Isl-1 transcription factor. Retinas from several different vertebrate species were analyzed at various developmental ages. Also, clonal derivation of cells in cultured embryonic chick retina was analyzed using retrovirus tracers of mitotic activity. Results. Müller cells are the last cell to withdraw from the cell cycle in vitro. Biochemical markers for these cells can be identified both before and after final mitosis. The appearance of Müller cell markers is associated with local appearance of certain neuronal markers. Radial cells that presumably function as migrational guides can be biochemically defined in primitive retinas prior to the appearance of other Müller cell markers. Conclusions. Müller cells mature in stages. Mitotically active cells early in development exhibit certain MC characteristics needed for tissue histogenesis. Functions necessary for mature tissue function, mainly arise after the final normal mitosis.