The family Nanopelagicaceae and the order Nanopelagicales were proposed as Candidatus taxa by Neuenschwander et al. (2018, ISMEJ, doi: 10.1038/ismej.2017.156). It contains two genera, Nanopelagicus and Planktophila, type strain for the family 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). Aerobic chemoheterotrophs. Cells are tiny, non-motile, and inhabit the plankton of freshwaters. The family is also known as acI or hgc1 (SILVA classification) from 16S rRNA based studies and is one of the most abundant microbes in freshwater lakes. Nanopelagicaceae can be recognized by the presence of the diagnostic oligonucleotide sequence 5’-AATGCGTTAGCTGCGTCGCA-3’ in the 16S rRNA gene (positions 852-872, E. coli numbering). The initial pure cultures were lost after a few propagations to fresh medium; no growing culture is available. The initial pure cultures were lost after a few propagations to fresh medium, they were isolated in sterile lake water amended with minimal carbon medium, vitamins and amino acids, no growth was observed in rich medium or on agar plates. No growing culture of Nanopelagicus sp. is available, while some Planktophila sp. strains are actively growing in autoclaved lake water (Kim et al. 2019, ISMEJ, doi: 10.1038/s41396-019-0432-x) or artificial media (Salcher et al., in review). None of the isolated strains were yet submitted to a culture collection because these bacteria are hard to maintain, i.e., they are very slowly growing, reach low densities in liquid culture, and do not grow on agar plates.