Type genome is Nanopelagicus abundans MMS-IIB-91 (GCF_002288305.1), which was isolated via dilution-to-extinction cultivation from Lake Zurich, Switzerland. Curved rods with lengths of 0.46±0.47 μm and diameters of 0.26±0.20 μm. The initial pure culture was lost after a few propagations to fresh medium; no growing culture is available. The initial culture grew well in sterile lake water amended with minimal carbon medium, vitamins and amino acids. Nanopelagicus abundans MMS-IIB-91 has a genome size of 1.16 Mbp and a genomic GC content of 40.2%. The genome is complete, consisting of a circular chromosome and contains 3 rRNA genes and 39 tRNAs. It is auxotrophic for reduced sulfur sources, several amino acids (methionine, lysine, ornithine, histidine, betaine) and several vitamins (B1, B5, B7, B12) and possesses rhodopsins. Members of the genus Nanopelagicus can be recognized by the presence of the diagnostic oligonucleotide sequence 5’-ACAAGAGGTTCGTCCGTCC-3’ in the 23S rRNA gene (positions 2669-2688, E. coli numbering). Basis for the assignment is a phylogenomic tree of 476 concatenated protein sequences, phylogenetic analysis of 16S rRNA, 23S rRNA, and rhodopsin genes, as well as average nucleotide and amino acid identities presented in Neuenschwander et al. (2018, ISMEJ, doi: 10.1038/ismej.2017.156). The closest cultivate relatives are Nanopelagicus limnes MMS-21-122 (GCF_002287885.2) and Nanopelagicus hibericus MMS-21-160 (GCF_002288005.1) with average amino acid identities of 81 and 80.7%, respectively, and average nucleotide identities of 77.1 and 76.2%, respectively.