The aim of this study is to gain insight into the time during the life history of a retinal neuron that it becomes committed to a particular phenotype. At this point, it is not possible to identify the time of commitment, but the time that differentiation begins can be identified. Bromodeoxyuridine labeling coupled with immunohistochemistry with a ganglion cell-specific antibody was used to fix the time of the beginning of ganglion cell differentiation relative to the time of mitosis in the developing chick retina. It was found that ganglion cells can begin to differentiate in less than 15 min after the end of mitosis. This suggests that the retinal ganglion cell fate may be determined before or during mitosis.