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Oligotrophic PYGV medium, inoculated with soils from Linnaeus Terrace (1600 m, Antarctica), yielded four aerobic actinomycetes with short rods, multiple and irregular septa and often motile buds. Cells were 1.0-2.8 x 1.0-3.0 pm and colonies were beige to pink. The isolates were nearly identical in physiological and biochemical tests. Three strains grew from 0 degrees C to 25-28 degrees C, but one was psychrophilic with a maximum growth temperature of 20 degrees C. Carbon sources utilized were a-glucose, a-galactose, lactose, sucrose or mannitol; malate, succinate, fumarate, pyruvate or glutarate were decarboxylated aerobically. Peptone and yeast extract were the preferred nitrogen sources. Nitrate was reduced aerobically or anaerobically. Cell walls contained mesodiaminopimelic acid, glutamate, alanine, glycine, galactose, glucose and ribose. Major fatty acids of strains AA-802, -824 -825 and -826(T) were n18:1, i16:0 and ai17:0. Major respiratory quinones were MK-S(H-4) and MK-8(H-4). Polar lipids were diphosphatidylglycerol, phosphatidylethanolamine and phosphatidylinositol. Phosphatidylglycerol was found in most strains. The DNA G+C contents were 68-70 mol%. In 16S rDNA analyses, similarity values obtained for 500 nucleotides from the 5’ terminus were > 99.5 %. Almost complete sequences from AA-826(T) and 825 were 99.9 % similar. Strain AA-826T belonged to a novel cluster of desert soil and rock isolates within the Geodermatophilaceae and was equidistantly related to members of Geodermatophilus and to a Blastococcus lineage. The four isolates represent a new genus, Modestobacter gen. nov., with Modestobacter multiseptatus sp. nov. as the type species. The type strain, Modestobacter multiseptatus AA-826T, was deposited in the DSMZ as DSM 44406(T).