No species of Dinophysis has yet been cultured in the full sense. To determine the photosynthetic capability in D. cf. acuminata, the dominant morphotype of the genus along the French Atlantic coast, we took material from natural populations, in some trials by pipetting out single cells, and in others using plankton assemblages enriched in D. cf, acuminata by size fractionation and reverse sedimentation. Inorganic carbon uptake was measured at temperatures from 11 to 23 degrees C and at a light intensity of 400 mu mol photons m(-2) s(-1). Total uptake was proportional to the number of cells and increased linearly with incubation time, the uptake rate showing a distinct maximum at 18 degrees C, while at 23 degrees C being only half this maximum. Photosynthetic capability was conclusively demonstrated for D. cf. acuminata, but mixotrophy remains a distinct possibility. Light microscopy of concentrated material showed numerous large, reddish-brown chloroplasts, which fluoresced yellow-orange. Their fine structure is rather unusual for dinoflagellates. No food vacuoles or residual bodies were seen, which suggests inability to ingest and digest prey directly, unless the required apparatus can be generated rapidly. Likewise, no feeding tubes were seen, but a tongue-like structure presenting microtubular bands was observed in the flagellar-pore region, which may be thus a simplified and reduced peduncle; this organelle, however, has not been seen protruded and active.