ABSTRACT.Gymnodinium sanguineum, Gyrodinium uncatenum, andCeratium furcaare large phototrophic dinoflagellates that commonly form red tides in the mesohaline portion of Chesapeake Bay during the summer. Examination of protargol‐stained specimens revealed that these dinoflagellates also feed heterotrophically as indicated by the presence of food vacuoles containing partially digested prey. Ingested prey were generally identified as nanociliates (≥20 μm) belonging to the oligotrich generaStrobilidiumandStrombidium; occasionally other small ciliates (e.g.Balanionsp. andMesodiniumsp.), dinoflagellates, and diatoms were observed in early stages of digestion. the percentage of these mixotrophs that had ingested prey was usually less than 20%, but approached 30% in some samples. Occurrence of food vacuoles inGymno. sanguineumwas positively correlated with ≤20 μm oligotrichous ciliate density; limited data forGyro. uncatenumsuggests a similar relationship, butC. furcafeeding was not related to nanociliate de