The ultrastructure and cytochemistry of secretion of the reproductive cyst wall in the ciliate Hyalophysa chattoni was analysed. The cyst wall is not secreted by exocytosis. Its primary material is chitin, which is secreted rapidly (< 30 min) at the surface of the cell. Transmission EM and ultracytochemistry reveal that the process is similar to chitin secretion and polymerization in yeast, fungi, and arthropods. The trichocysts are few in the tomont stage and are not involved in encystment.