Traces of microscopic filaments of the cyanophytic algaGirvanellahave been found in the inferred regressive Lower Ordovician limestone sequence at Hälludden, N. Öland. The algae and their mode of occurrence, as aggregates or as encrustations, are described. Their vertical range is discussed in relation to water depth and water energy. A correlation between the presence ofGirvanellaand limited water depth is demonstrated. The distribution ofGirvanellathus supports the idea of a shallowing in theA. expansuszone in the upper part of the section. An observed association between sponge spiculae and encrusting blue-green algae, is discussed briefly.