An aerobic, Gram-positive, non-motile, non-spore-forming, rod-coccus-shaped bacterium, strain 5317S-21 T , was isolated from an air sample from Suwon city, Republic of Korea. The isolate was able to grow within a pH range of 5.0-9.0 and a temperature range of 5-35 6C and it tolerated up to 2 % (w/v) NaCl. The cell-wall peptidoglycan contained meso-diaminopimelic acid as diagnostic diamino acid. The predominant isoprenoid quinone was MK-8(H 4 ). The major polar lipids were phosphatidylinositol, phosphatidylethanolamine and diphosphatidylglycerol; phosphatidylglycerol and several unknown phospholipids were also detected. Mycolic acids were absent. The only whole-cell sugar was glucose. The major cellular fatty acids were iso-C 16 : 0 , C 17 : 1 v8c and iso-C 15 : 0 . 16S rRNA gene sequence analysis indicated that strain 5317S-21 T was related phylogenetically to members of the genus Knoellia, with 97.4 % sequence similarity to the type strains of Knoellia sinensis and Knoellia subterranea. The G+C content of the genomic DNA of strain 5317S-21 T was 73 mol%. Levels of DNA-DNA relatedness between strain 5317S-21 T and the type strains of Knoellia sinensis and Knoellia subterranea were 37 and 41 %, respectively. It was concluded that strain 5317S-21 T represents a novel species of the genus Knoellia, for which the name Knoellia aerolata sp. nov. is proposed. The type strain is 5317S-21 T (5KACC 20583 T 5DSM 18566 T ).The genus Knoellia is a member of the family Intrasporangiaceae within the class Actinobacteria and contains organisms that are Gram-positive, non-motile, irregular rods or cocci. The peptidoglycan is of type A1c, the predominant menaquinone is MK-8(H 4 ) and the major fatty acids are iso-C 15 : 0 , iso-C 17 : 0 , iso-C 16 : 0 and anteiso-C 17 : 0 . The major polar lipids are phosphatidylethanolamine (PE), phosphatidylinositol (PI) and diphosphatidylglycerol (DPG). Currently, the genus is represented by two recognized species, Knoellia sinensis and Knoellia subterranea (Groth et al., 2002).An air sample was collected in Suwon city, Republic of Korea, using an MAS-100 air sampler (single-stage multiple-hole impactor; Merck), which contained Petri dishes with R2A agar (BBL) supplemented with 200 mg cycloheximide ml 21 (Sigma). Strain 5317S-21 T was isolated after incubation for 5 days at 30 u C. The isolate was subcultured on R2A medium at 30 u C and maintained on the same medium.Genomic DNA was extracted using a DNeasy Tissue kit (Qiagen) following the manufacturer's instructions. Sequencing of the 16S rRNA gene was performed as described by Kwon et al. (2003). Multiple alignments were performed by using the program CLUSTAL W (Thompson , 1994). Evolutionary distances were calculated using the Kimura two-parameter model (Kimura, 1983). Phylogenetic trees were constructed with neighbour-joining (Saitou & Nei, 1987) and maximum-parsimony methods (Fitch, 1971) Fig. 1. According to the neighbour-joining tree ( Fig. 1), strain 5317S-21 T clustered with the two recognized species of the genus Knoellia with a 68...