Extracts of Euglena gracilis cells contain high levels of elongation factor (EF)-Ts (EF-Tschl) activity which can be assayed by measuring the rate of exchange of GDP with Escherichia coli EF-Tu . GDP. The appearance of EF-Ts activity in E. gracilis cells is light-stimulated, suggesting that the EF-Ts is required for chloroplast function. However, based on experiments with a mutant of E. gracilis lacking chloroplast DNA, as well as studies on the effect of antibiotics on EF-Ts synthesis, it is concluded that the EF-Tschl gene is nuclear-coded.