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Abstract
A new Rhoicosphenia species, R. tenuis Levkov & Nakov is described from Lake Ohrid. It is characterized by its narrow linear valves (valve width up to 4.5 μm), which differentiate it from the otherwise similar R. curvata. This species has an ultrastructure typical of the rest of the freshwater representatives of the genus Rhoicosphenia. The concave valve possesses the typical raphe system, while the convex valve has a reduced raphe system. A well developed pseudoseptum is present at the poles, and both valves possess an apical pore field at their foot pole. According to valve morphology and ultrastructure, R. tenuis is closer to R. curvatathan to Rhoicosphenia macedonica, another species described from Lake Ohrid,