We previously isolated five strains of putative lactobacilli from the faeces of a thoroughbred horse (a 4-year-old male). Of the five strains, four were identified as members of existing Lactobacillus species; however, sequence analysis of the 16S rRNA gene revealed that the fifth isolate, DI70(T), showed approximately 97 % identity (1325/1366 bp) with the type strain of Lactobacillus delbrueckii. Therefore, we considered the possibility that DI70(T) represents a novel species of the genus Lactobacillus. Cells of strain DI70(T) were Gram-stain-positive, catalase-negative, non-spore-forming, non-motile rods. In phylogenetic trees constructed on the basis of 16S rRNA gene sequences, strain DI70(T) formed a subcluster in the L. delbrueckii phylogenetic group and was closely related to L. delbrueckii, Lactobacillus crispatus and Lactobacillus jensenii. However, analysis of DNA-DNA relatedness showed that DI70(T) was genetically distinct from its phylogenetic relatives. The isolate also exhibited distinct biochemical and physiological characteristics when compared with its phylogenetic relatives. It required anaerobic conditions for growth on agar medium. The results indicate that isolate DI70(T) indeed represents a novel species of the genus Lactobacillus, for which we propose the name Lactobacillus equicursoris sp. nov. The type strain is DI70(T) (=JCM 14600(T) =DSM 19284(T)).