Many of the mitochondria of Tetrahymena thermophila are localized in the cell cortex in regular, identifiable patterns. Two different mitochondrial patterns are seen; whether a cell expresses one or the other type apparently depends upon nutrient conditions in the culture and not upon other factors tested. Consistent associations between cortical mitochondria and certain of the cortical microtubular bands are seen at the light-microscopic level. Electron-microscopic observations confirm this and, furthermore, identify ultrastructural associations between cortical microtubules and the cortically located mitochondria. It appears that the cortical microtubular arrays serve as a guide for the localization (and thus patterning) of the cortical mitochondria.