In summer to autumn of 2008, a recently described thecate mixotrophic dinoflagellate, Fragilidium duplocampanaeforme Nézan et Chomérat, occurred in Masan Bay, Korea, where it frequently contained bright-orange fluorescent inclusions. Using cultures of F. duplocampanaeforme isolated from Masan Bay, we investigated feeding, digestion, and prey specificity of this mixotroph. F. duplocampanaeforme fed exclusively on Dinophysis spp. when offered a variety of prey including dinoflagellates, a raphidophyte, a cryptophyte, a ciliate, and diatoms separately. In addition, F. duplocampanaeforme had allelopathic effects on other organisms, including cell immobilization/motility decrease (in Dinophysis acuminata, D. caudata, D. fortii, D. infundibulus, Gonyaulax polygramma, Heterocapsa triquetra, and Prorocentrum triestinum), breaking of cell chains (in Cochlodinium polykrikoides), cell death (in Prorocentrum minimum), and temporary cyst formation (in Scrippsiella trochoidea). F. duplocampanaeforme engulfed whole Dinophysis cells through the sulcus. About 1 h after ingestion, F. duplocampanaeforme became immobile and shed all thecal plates. The ecdysal cyst persisted for ∼7 h, during which the ingested prey was gradually digested. These observations suggest that F. duplocampanaeforme may play an important role in the Dinophysis population dynamics in the field.