Transfer of exponentially growing cells of the ciliated protozoan Tetrahymena pyriformis into a non-nutrient medium induces a pronounced phosphorylation of a single small subunit ribosomal protein, S6. Within the first hour of starvation S6 is converted completely from the non-phosphorylated state found in exponentially growing cells into a highly phosphorylated state. This state, however, is normally maintained only for a few hours, after which S6 is dephosphorylated. In the subsequent period of starvation S6 normally remains non-phosphorylated. In a series of experiments the patterns of phosphorylation and dephosphorylation of S6 were analysed in relation to the changes in the metabolism of RNA that occur during the transition from growth to starvation conditions. These experiments showed that the level of S6 phosphorylation correlated closely with the rate of RNA degradation and thus, since changes in the cellular content of RNA reflect changes in the content of ribosomes, suggests that phosphorylation of S6, apart from possible functions in the process of translation, may be involved in the regulation of ribosome catabolism in Tetrahymena pyriformis.