Dinoflagellates are one of the most abundant and diverse components of marine plankton. Many of them, if not all, are infected by the endoparasitoid Amoebophrya ceratii (Syndiniales, Marine Alveolate Group II), an early emerging dinoflagellate group. As these parasitoids frequently display narrow host ranges, they are believed to be as diversified as their hosts and are composed of cryptic species. Infections last 2–4 days and end with the host’s death and the release of a transient fast-swimming colony (the vermiform) that fragments within a few hours into hundreds of infective cells (dinospores). Due to this exponential growth, A. ceratii exerts striking potential on top-down control of planktonic dinoflagellate blooms in natural waters. Both A. ceratii and their dinoflagellate hosts share the same common ancestor, a red myzozoan microalga. A. ceratii is rather atypical by having lost its plastid and having an extremely reduced mitochondrial DNA. It can be cocultivated with its host but not cryopreserved.