Water-soluble CdTe quantum dots (QD600) were synthesized by hydrothermal route. The uptake of QD600 in living cells of euglena gracilis was studied. The luminescence spectra show that the QD600 were bound in cells after incubation. The fluorescence images, measured by confocal laser scanning microscope, demonstrate that the QD600 can penetrate into the cells. The amount of cell-bound QD600 increased with incubation time of QD600 at 25 Celsius deg., but no detectable QD600 were found in the cells when the incubation temperature was 4 Celsius deg., it was suggested that the QD600 were actively taken into the cells by endocytotic pathway. These results indicate that the water-soluble CdTe quantum dots have the potential in the application of bio-labeling.