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Five strains that were morphologically identified as Planktolyngbya circumcreta were isolated from three geographical regions in China. The strains were phylogenetically and taxonomically characterised based on a polyphasic approach that merged morphological and genetic characteristics, as well as ultrastructure. The ultrastructure of these strains showed a similar arrangement of thylakoids, with four to eight parallel layers that were parietally distributed, suggesting that they belonged to the order Synechococcales, family Leptolyngbyaceae. As inferred from 16S rRNA gene sequence analysis, these pure cultures share >= 98.5% similarity and were grouped into a unique and robust cluster with high bootstrap values. This unique cluster was separated from a clade composed of two strains of Planktolyngbya limnetica, which belonged to the type species of the genus Planktolyngbya. Thus, we transferred these five P. circumcreta strains to the new genus Limnolyngbya. The species Limnolyngbya circumcreta nov. comb. was designated as the type of the genus.