ENDOGENOUS RHYTMS AND SEASONAL OCCURRENCE OF DINOFLAGELLATES. - Although circannual rhythms of ecological characters have been widely reported in dinoflagellates, the role played by internal clocks in their occurrence and abundance in nature still remain elusive. As a first approach, we made a comparative analysis of the temporal structures of: a) endogenous circannual rhythms in growth rate under constant laboratory conditions and b) annual cycle of occurrence-abundance in nature in Galician water (NW Spain), for three dinoflagellates species (Prorocentrum micans Ehrenberg. Gonyaulax excavata Braarud, and Schrippsiella trochoidea Stein) by means of COSINOR rhythmometrical analysis techniques. A statistically significant concordance between the achrophases of both rhythms for all species was reported, suggesting that endogenous rhythms influence the annual cycle of dinoflagellates in nature. We propose a model to integrate the effects of environmental and stochastic factors, and internal clocks on the modulation of the circannual occurrence of dinoflagellates in nature.