ABSTRACTThe dinoflagellateAmphidinium cryophilumsp. nov. is one of the few gymnodinians to be studied at the ultrastructural level. It resembles other dinoflagellates in the structure of the nucleus, trichocysts, storage materials, flagella, mitochondria, and microbodies. Other features ofA. cryophilumless commonly observed in related organisms include a network of small interconnected vesicles, a system of large, peripheral vacuoles, chloroplasts bound by two rather than three membranes, an accumulation body, thylakoid‐associated plastoglobuli, a vesiculated nuclear envelope, a complex tubular pusule, striated flagellar collars, collared pits, and a peduncle. The occurrence of a peduncle, a structure implicated in phagotrophy, in this autotrophic organism is noteworthy. The ultrastructure of the peduncle ofA. cryophilumdiffers significantly from that reported in another dinoflagellat