Synchronous cultures of a high temperature strain of Chlorella pyrenoidosa have been used to determine the activity of the TMP kinase during cellular development. It was observed that the apparent enzyme activity was closely correlated with the rate of DNA biosynthesis. During the period of nuclear division, when DNA synthesis is at its maximum apparent TMP kinase activity is highest. The maximum in enzyme activity, however, slightly precedes the peak in DNA synthesis. These results support the hypothesis that TMP kinase activity is a factor controlling the rate of DNA biosynthesis. It appeared that the dramatic shifts in the level of TMP kinase activity were a result of variations in the rate of enzyme synthesis rather than control of enzyme activity by small molecule inhibitors or activators. / Ph. D.