Several strains of Pseudo-nitzschia delicatissima were isolated from the northwestern Mediterranean Sea and compared using light and electron microscopy, phylogenetic analyses of internal transcribed spacer rDNA, together with studies of their mating system, estimations of growth rates and reduction in cell size with time. Morphological and phylogenetic analyses identified all strains as Pseudo-nitzschia delicatissima, but in the phylogenetic analyses they fell into two genetically distinct clades. Studies of fine structural morphological characteristics by electron microscopy did not allow discrimination of the two clades. Mating experiments showed successful sexualisation among strains of clade B/1; whereas, mating attempts in clade A/2 and between clade A/2 and clade B/1 were unsuccessful under the examined laboratory conditions. The morphological characteristics of the daughter generations F1 and F2 were analyzed and abnormalities in the F2 generation described. We describe for the first time the rapid size reduction in the F2 generation. Culture studies showed physiological differences between the two clades; strains from clade A/2 showed a tendency for higher growth rates and slower reduction in cell size than strains from clade B/1. Morphological and phylogenetic studies comparing strains from different parts of the world showed P. delicatissima as a cryptic species complex comprising at least two different genotypes, both apparently cosmopolitan. The two genotypes were shown to occur sympatrically in the Mediterranean Sea, and we hypothesize that the genotypes were most likely separated by reproductive barriers associated with sexualisation occurring at different temperatures and hence different seasons. The molecular and physiological differences allow us to describe one of the genotypes, clade B/1, as a new species, Pseudo-nitzschia arenysensis sp. nov. Differences in growth capabilities of the cultures were observed in relation to cell size and physiological status and showed a nonlinear relationship between cell size and growth. Possibly, such capability plays a key role in the occurrence and dynamics of phytoplankton algae.