The marine diatom Pseudo-nitzschia galaxiae Lundholm et Moestrup has been recently described from Mexican and Australian plankton. In this paper, we illustrate the considerable morphological variability of the species in the Mediterranean Sea and present first evidence of its toxicity. In addition to lanceolate cells 25–41 μm long, which fit the original description of the species, markedly larger (<82 μm) and smaller (>10 μm) specimens are commonly recorded. Cells of the largest size have almost parallel valve margins, while smaller specimens have extremely short rostrate ends and do not form colonies. Despite remarkable differences in shape and size, the typical ultrastructure of the species was observed for the different size classes in culture and in natural samples. In culture, cell length decreased at a rate of 1.1–2.1 μm per month. Liquid chromatography–mass spectrometry (LC–MS) analyses revealed the presence of domoic acid (DA) at very low levels in two of seven strains analyzed. LSU rDNA analysis confirmed the identity of the species and showed a very low genetic variability for the strains from the Gulf of Naples, with no relationships with size and overall shape of the cells. A relatively high number (53) of Pseudo-nitzschia sequences were considered in the phylogenetic analysis, yet the relationships among species remain unclear, probably in relation with a recent speciation process in the genus. In natural samples, P. galaxiae populations of different cell sizes occurred at different times over the year, with smaller cells found in winter and early spring, and medium and larger cells peaking in late spring–summer. The maximum concentration value in the Gulf of Naples was recorded in May 1985 (9.4×106 cells l−1). From the analysis of a high number of both natural and culture samples, it is concluded that size and shape variations are indicative of different stages of the life cycle of P. galaxiae, which exhibit a synchronized and seasonal occurrence at the interannual scale.