A bacterial strain, S21BT, was isolated from Altamira Cave (Cantabria, Spain). The cells were Gram-negative, short rods growing aerobically. Comparative 16S rRNA gene sequence analysis revealed that strain S21BT represented a separate subline of descent within the family 'Aurantimonadaceae' (showing 96% sequence similarity to Aurantimonas coralicida) in the order Rhizobiales (Alphaproteobacteria). The major fatty acids detected were C16:0 and C18:1omega7c. The G+C content of the DNA from strain S21BT was 71.8 mol%. Oxidase and catalase activities were present. Strain S21BT utilized a wide range of substrates for growth. On the basis of the results of this polyphasic study, isolate S21BT represents a novel species of the genus Aurantimonas, for which the name Aurantimonas altamirensis sp. nov. is proposed. The type strain is S21BT (=CECT 7138T=LMG 23375T).