ABSTRACT: The in situ growth rate of phytoplanktonis defined as the potential growth rate without adjustment for losses due to cell mortality when it is measured by the cell cycle approach. In contrast, the method that assesses the cellnumber variation of the population in diffusion chambers takes cell mortality into account. We compare the 2 methods for natural blooms of dinoflagellates (Alexandrium minutum, Alexandrium taylori, Ceratium furca, Dinophysis sacculus,Gymnodinium pulchellum and Gyrodinium corsicum) grown in various situations and locations and data reported elsewhere. The in situ potential growth rates showed great intraspecific variability. The use of the differences between the 2methods as a measure of in situ cell lysis is discussed.