Phytoplankton samples were collected along the north coast of the Antarctic peninsula, the Weddell Sea and the Gerlache Strait, in different seasons in 1985, 1986, 1999, and 2000–2002. Chaetoceros castracanei Karsten, a fairly rare species was present in the samples; its morphology, taxonomy and distribution have not been recently investigated. This species occurred mainly from January to April, and was recorded with relatively high abundances in the Weddell Sea (up to 9.5=10 4 cells l -1 ). C. castracanei forms straight or slightly curved, short to long chains, with the terminal and intercalary setae pointing in the same direction. The apertures are very reduced, centrally constricted, and there are numerous chloroplasts in the cells including the setae. Electron microscopy studies showed that valves are randomly perforated by round poroids, costae on the valve are absent, and an excentric rimoportula is present in every valve of the chain; the setae are circular in cross section at the base, but become four-sided distally. Setae have perpendicular rows of elongate poroids, spines are arranged in spirals along the setae and the tips are very pointed. The intercalary setae are fused together in sibling valves. Chaetoceros impressus Jensen et Moestrup, recently described from Danish waters, appears to be conspecific with C. castracanei, and consequently we propose it as a synonym of the latter. The known distribution of Chaetoceros castracanei may be broadened to include Antarctic, subantarctic and Danish waters, and also possibly more temperate regions.