The ultrastructure of vegetative cells and cysts of Azotobacter chroococcum shows that these two morphological forms are distinctly different. The cyst is surrounded by a multi-layered envelope consisting of the following structural elements: an irregularly outlined exocystorium and a cyst coat made up of an outer dense and an inner less dense layer. The cell inside the cyst coat shows essentially the same structural features as the normal vegetative cell. The cyst thus bears no true resemblance to the endospore of the Grampositive bacillus.