The genus Nanopelagicus as well as the family Nanopelagicaceae and order Nanopelagicales was proposed as Candidatus by Neuenschwander et al. (2018, ISMEJ, doi: 10.1038/ismej.2017.156). Three strains were isolated from Lake Zurich, Switzerland, that were proposed as Nanopelagicus abundans, Nanopelagicus limnes and Nanopelagicus hibericus, type strain for the genus is Nanopelagicus abundans MMS-IIB-91 (GCF_002288305.1). Basis of the assignment is a phylogenetic tree of 48 conserved concatenated proteins of >100 complete genomes of all orders of Actinobacteria in Neuenschwander et al. (2018, ISMEJ, doi: 10.1038/ismej.2017.156). The closest cultivated relatives are Planktophila sp. with average amino acid identities of 62-63.9% and average nucleotide identities of 65.9-68.1%. Aerobic chemoheterotrophs. Cells are tiny, non-motile, and inhabit the plankton of freshwaters. The genus is also known as acI-B1 or hgc1 (SILVA classification) from 16S rRNA based studies and is one of the most abundant genera in freshwater lakes. 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). The initial pure cultures were lost after a few propagations to fresh medium; no growing culture is available. The initial cultures grew well in sterile lake water amended with minimal carbon medium, vitamins and amino acids.