Strains T138021-75(T), Pg19 and Pg20 (taxon 25 of Bisgaard) were isolated from guinea pigs and characterized. Strains T138021-75(T) and Pg20 showed identical 16S rRNA gene sequences and were distantly related to the published strain P224 with the highest 16S rRNA similarity of 98.6 %. These two strains showed 97.8 % sequence similarity with the type strain and other strains of Mannheimia glucosida and 97.3 % similarity with the type strain of Mannheimia varigena, but <97 % similarity with all other type strains of the genus Mannheimia, including Mannheimia haemolytica (96.9 %). Phylogenetic analysis of rpoB gene sequences showed that strain P224 had a distant position (89.9 % gene sequence similarity) compared with the three other strains (T138021-75(T), Pg20 and Pg19), which had identical gene sequences. These three novel strains also shared identical recN gene sequences. Phylogenetic analysis of the recN gene sequences showed a close relationship between the three novel strains and strain P224. The DNA-DNA reassociation value between strain T138021-75(T) and P224 was 81.6 % and 40.3 % between strain T138021-75(T) and the type strain of M. glucosida. Based on the DNA-DNA reassociation data, strain T138021-75(T) belonged to a separate species that was closely related to strain P224. Strain P224 differed from strains T138021-75(T), Pg20 and Pg19 in the following phenotypic characteristics: activity of ornithine carboxylase, hydrolysis of glycosides, and acid formation from maltose, dextrin, melibiose and raffinose, as well as reactions for α-galactosidase and β-xylosidase. Whole genome similarity calculations based on recN gene sequences showed that strains T138021-75(T) and P224 were related at the species level (0.932), whereas 16S rRNA and partial rpoB gene sequence comparisons showed a more divergent position of strain P224 compared with the novel strains, including a different host of isolation. The results showed that the three strains of taxon 25 represent a novel species for which the name Mannheimia caviae sp. nov. is proposed. The type strain, T138021-75(T) ( = CCUG 59995(T) = DSM 23207(T)) was isolated from purulent conjunctivitis in guinea pigs. Previous publications have documented both ubiquinones and demethylmenaquinone to be present in the type strain. The G+C content of the DNA of the type strain has been found to be 41.4 mol% (T(m)).