ABSTRACT: This study explores vertical heterogeneities of in situ division rates (μ) of 3 species of Dinophysis and discusses its implications in modelling the population dynamics of these species. Based on a post-mitotic index approach, estimates of μ from vertical net-haul (integrated) samples (μ_int) were compared with those from a single depth (oceanographic bottle samples) (μ_z) at the cell maximum during 2 daily cell cycle studies in the Galician Rías Baixas (NW Spain). Additionally, vertical profiles of μ_z were obtained during a field survey in Ría de Vigo. A 2-fold difference was observed between estimates of μ_z from the cell maxima and μ_int from integrated samples under stratified conditions. Differences were much larger when the minimum estimates of μ values from different depths (μ_min) were compared within single vertical profiles. An exponential growth model was applied to simulate the dynamics of a D. acuminata population during a daily cycle in June 1994. Results show that actively dividing cells of a target species may be restricted to narrow layers of the water column. Estimates of μ at different relevant depths during cell cycle studies may be key to determining whether if increased numbers of a target species are due to in situ growth or to the balance of imports and exports.