Summary Spherical, dense particles, up to 2 μ in diameter, have been isolated from the ciliate, Tetrahymena pyriformis. By a combination of chemical and enzymatic analysis the particles have been shown to consist primarily of a hydrated mixed pyrophosphate of calcium and magnesium. The same particles have also been located within the cell by histochemical means and identified enzymatically. The possible function of the particles was discussed in connection with their composition and their behaviour during cell division. It is suggested that inorganic pyrophosphate may serve as a phosphagen in Tetrahymena.