Taxonomic studies were performed on seven strains isolated from microbial mats in Antarctic lakes of the McMurdo Dry Valleys. Phylogenetic analysis based on 16S rRNA gene sequences indicated that these strains are related to the genus Flavobacterium; sequence similarity values with their nearest phylogenetic neighbours ranged from 97.0 to 98.7 %. Results of DNA-DNA hybridization and comparison of repetitive extragenic palindromic DNA-PCR fingerprinting patterns revealed that these strains are members of two distinct species. Genotypic results, together with phenotypic characteristics, allowed these species to be differentiated from related recognized Flavobacterium species. The isolates are psychrophilic, Gram-negative, chemoheterotrophic, rod-shaped cells. Their whole-cell fatty acid profiles are similar and include C(15 : 0), C(15 : 0) anteiso, C(15 : 0) iso, C(15 : 1)omega6c, C(16 : 0) iso, C(16 : 0) iso 3-OH, C(16 : 1) iso and summed feature 3 (which comprises C(15 : 0) iso 2-OH and/or C(16 : 1)omega7c) as the major fatty acid components. On the basis of these results, two novel species are proposed: Flavobacterium fryxellicola sp. nov., consisting of three strains with LMG 22022(T) (=CIP 108325(T)) as the type strain; and Flavobacterium psychrolimnae sp. nov., consisting of four strains with LMG 22018(T) (=CIP 108326(T)) as the type strain. DNA G+C contents of F. fryxellicola and F. psychrolimnae are 35.2-35.9 and 33.8-34.5 mol%, respectively.