1. The green bodies in the intestinal epithelial cells of three species of Chaetopteridae, each representing a different genus, have been examined.
2. They are very similar in the three cases and resemble those of Chaetopterus variopedatus.
3. They are frequently motile and flagellated when separated from the intestinal tissue.
4. They represent an infection of an organism which spreads in a flagellated condition.
5. The organism is a flagellate probably belonging to the family Chrysocapsinae.
6. The name Chrysocapsa chaetopteri is proposed for it.