We have previously shown that the adult thymus contains three subsets of gamma delta T cells that can be defined by the expression of THY-1 and heat-stable antigen (HSA). In this study, the number of cells in each of these thymic gamma delta populations was investigated at different stages throughout life. In adult mice, the populations stay relatively constant, however, in contrast, there were major variations in them early in development. It was shown that only two of the gamma delta populations were present in the prenatal thymus, a major population of Thy-1+ HSA - cells, and a smaller population of Thy-1+ HSA+ cells. However, after birth, most of the Thy-1+ HSA - cells appear to loose the Thy-1 antigen, forming the third population of HSA - Thy-1 - cells. The adult configuration of populations appeared to be established within the first week after birth. Therefore, whereas the gamma delta populations stayed relatively constant from this time point onwards, there were major variations early in development. Throughout life, most gamma delta thymocytes are CD4- CD8-, however, in the neonatal thymus, there are some CD+ and CD+ gamma delta thymocytes, and these are contained in the Thy-1+ HSA - population.