ABSTRACT: Growth and grazing responses of the mixotrophic dinoflagellate Fragilidium subglobosum were studied as a function of prey concentration, prey species, and light intensity in laboratory cultures. In monospecific (exclusively phototrophic) culturesthe growth rate of F. subglobosum was 0.16 d^-1 (doubling time 4.3 d) at a light intensity of 45 µmol photons m^-2 s^-1. In cultures supplied with the phototrophic dinoflagellate Ceratium tripos at a similarlight intensity, the growth rate of F. subglobosum reached a maximum level of 0.5 d^-1 (doubling time 1.4 d) at a prey concentration of ca 10 C. tripos cells ml^-1. The functional response of F. subglobosumfollowed a Holling type I functional response. At prey concentrations which resulted in maximum growth rate, growth yield equalled ca 40%. However, at prey concentrations which led to lower growth rates, growth yield exceeded 100%, indicating that fooduptake by F. subglobosum stimulated photosynthesis at low prey concentrations. When C. tripos cells were added in excess, growth and ingestion rate of F. subglobosum increased with light intensity within the studied range (9 to 45µmol photons m^-2 s^-1). Growth rates of F. subglobosum were higher with C. tripos as food than with C. furca and C. fusus.