In the ciliate, Tetrahymena pyriformis, the population of cell organelles shows variation during the culture cycle and when the cells are subjected to various treatments. New cell constituents may appear and the fine structure of mitochondria and peroxisomes may change. Such dynamic structural changes are described and reviewed in relation to physiological and biochemical studies on Tetrahymena. The cell organelles discussed are about I tam in diameter or smaller~ reference is also made to the nuc[eotar organization which is the most sensitive indicator of the physiologica~ state of the ceils. The presumed distribution of the various cell organelles in subcellular fractions of Tetrahymena is mentioned, since dependent on the pretreatment of the cells, especially the mitochondrial and peroxisomal fractions may be contaminated to varying degrees with other cell organelles~The purpose of the review is to demonstrate that a correlation exists between the structure and function of cell organelles, especially mitochondria and peroxisomes, and that the overall fine structure of a cell reflects its physiological state.