The Dsrc28C gene encodes two major proteins, p66 and p55, each of which contains a tyrosine kinase domain. Using monoclonal antibodies we have completed a detailed investigation of the spatial expression of Dsrc28C proteins during embryonic and larval development. Differentiation of a number of embryonic tissues is accompanied by the induction of Dsrc28C expression. With the exception of the developing salivary glands which express high levels of p66, developing tissues express the p55 form of Dsrc28C. Notable examples are cells of the and peripheral nervous systems which express p55 from the early stages of neurogenesis through the remainder of embryogenesis and pole cells which transiently express p55 during portions of embryonic stages 10 and 11. Nervous system expression includes the cell bodies and neuronal fibers of the central nervous system, the anterior sensory organs, and the peripheral sensory neurons. During larval development, p55 levels within the central nervous system remain high but substantial changes in the pattern of expression take place. p55 gradually disappears from the neuronal fibers of the central nervous system and from embryonic cell bodies. During the third larval instar, the birth of immature neuroblasts within the ventral and midbrain ganglia, but not within the optic ganglia, is marked by a transient high level of p55 expression. All imaginal cells that have been observed within the larva express the p66 protein. The patterns of expression that we have noted suggest that expression of the p55 form of Dsrc28C protein is an early event in the differentiation of neuronal cells, while expression of the p66 form is characteristic of cells committed to ectodermal cell differentiation.