The teguments of four cestode species and the intestines of their six fish hosts inhabited by bacteria have been investigated with electron microscopy. Most of the symbiotic bacteria have sizes very similar to nanobacteria. The features of different morphological forms of nanobacteria have been identified. It has been noted that the digestive transport surfaces of helminths and their fish hosts are inhabited by nanobacteria of various morphological forms. A study of the ultrastructure of the only nanobacterium suggests that these microorganisms are fully viable, contain genetic substance, and are capable of reproduction.