Azadinium caudatum is a poorly known planktonic dinoflagellate, which mainly attracts attention due to the occurrence of many toxigenic species in the genus Azadinium. The availability of two Scottish isolates of A. caudatum var. margalefii allowed for the first time a detailed analysis of biological and physiological traits of the species, specifically on the potential presence of azaspiracid toxins (AZAs). With a mean overall swimming speed of 85 μm s-1 and regularly interspersed high-speed jumps, A. caudatum var. margalefii exhibited a similar swimming behaviour to other species of the genus. Cells had a single, large, highly reticulated chloroplast with a typical pigment composition for peridinin-containing dinoflagellates. No pyrenoids were visible under light microscopy. A round to ovoid nucleus was centrally located. Cell division was by desmoschisis, i.e. the parent theca was shared by the daughter cells. Growth rate μ ranged from 0.07 to 0.32 day-1 and was reduced at lower temperatures. With growth rate becoming light saturated at intensities of about 40 μE m-2 s-1 and a half-saturation light intensity of about 13 μE m-2 s-1, A. caudatum var. margalefii appears to be relatively low-light adapted. Neither strain produced any known AZAs in measureable amounts. © 2014 © 2014 Taylor & Francis.