Euglena gracilis growing in media with different chemical composition showed changes in its nucleic acid and protein content. The cell density reached in the stationary phase of growth in synthetic medium is higher compared with that of the complex medium. There is an inverse relationship between cell density in the stationary phase of growth and the protein and RNA content of the cells. This phenomen is interpreted by a selective stimulation of the division process over the growth process. Euglena gracilis cultivated in complex and synthetic media in the dark have different amounts of high molecular weight RNA during the stationary and logarithmic phases of growth. The amount of transfer RNA did not change in any condition. The base composition of the high molecular weight RNA differs when the cells were cultivated in complex or synthetic medium.