Asterionella is a long, thin, rod-like araphid diatom.  It has heteropolar ends, meaning that each of its ends is distinctly different and the diatom is asymmetrical on the axis perpendicular to its length. Striae are small and numerous, wrapping around the valve with somewhat irregular spacing.  In processed samples the cells will be individual, but in their natural state they form star-shaped colonies joined at one pole and spiking outward.