A novel Bacteriastrum (Chaetocerotaceae, Bacillariophyta) species, B. parallelum, was Studied with light and electron microscopy using both cultured and natural material from the Gulf of Naples (Mediterranean Sea). This is the only non-colonial Bacteriastrum species described. Cells are very small (diameter 3-10 mu m), with 5-12 chloroplasts, and with thin setae arising from the valve face rim and forming an acute angle with the pervalvar axis. Cells are heterovalvate. with setae of the anterior valve almost parallel to those of the posterior valve. The species is common in spring samples from the Gulf at times reaching concentrations of 5 x 10(6) Cells l(-1). We also examined natural and cultured material of the type species of the genus. B. furcatum Shadbolt. Our observations provide evidence that in this species the setae of the anterior terminal valve consistently have a different shape and orientation from those of the posterior terminal valve. Therefore, Bacteriastrum furcatum should be moved from the section Isomorpha, where it has been placed, to the section Sagittata, which contains the heteropolar taxa.