Planktonic members of most algal groups are known to harbor intracellular symbionts, including viruses, bacteria, fungi, and protozoa. Among the dinoflagellates, viral and bacterial associations were recognized a quarter century ago, yet their impact on host populations remains largely unresolved. By contrast, fungal and protozoan infections of dinoflagellates are well documented and generally viewed as playing major roles in host population dynamics. Our understanding of fungal parasites is largely based on studies for freshwater diatoms and dinoflagellates, although fungal infections are known for some marine phytoplankton. In freshwater systems, fungal chytrids have been linked to mass mortalities of host organisms, suppression or retardation of phytoplankton blooms, and selective effects on species composition leading to successional changes in plankton communities. Parasitic dinoflagellates of the genus Amoebophrya and the newly described Perkinsozoa, Parvilucifera infectans, are widely distributed in coastal waters of the world where they commonly infect photosynthetic and heterotrophic dinoflagellates. Recent work indicates that these parasites can have significant impacts on host physiology, behavior, and bloom dynamics. Thus, parasitism needs to be carefully considered in developing concepts about plankton dynamics and the flow of material in marine food webs.