Zoospores of the dinophytePaulsenella cf.chaetoceratis, parasitizing the marine diatomStreptotheca thamesis, attach to the girdle region of the host and drive a peduncle into the cell interior. The peduncle consists of a non-cytoplasmic “crook”, a cytoplasmic feeding tube, and a presumably cellulosic sheath around the proximal part of the feeding tube. The crook seems to be used for attachment and penetration of the host. The mobile feeding tube induces shrinkage of the host vacuoles and takes up the complete host cytoplasm within less than 1 h. Phagocytosis depends on an intact host plasmalemma, which is not penetrated by the feeding tube. The trophic phase ends with retraction of the feeding tube. While the food is digested within a large vacuole, the trophont transforms into a thick-walled primary cyst. After about 12 h the primary cyst divides to form 3 or 4 secondary cysts. Finally, about 24 h after attacking the host, each secondary cyst releases two zoospores which may be again ready for infection within 1 h, without passing through any intermediate stage. The developmental times (above referred to 20 °C) are highly dependent on the temperature and can vary considerably, even between sister cells.