Biochemical and molecular genetic studies were performed on three isolates of an unknown Gram-positive, catalase-negative and rod-shaped organism isolated from the lungs and liver of two beaked whales. The organisms were tentatively identified as Lactobacillus spp. based on cellular morphology and biochemical tests. 16S rRNA gene sequencing studies confirmed the provisional identification of the novel isolates as members of the genus Lactobacillus, but the isolates did not correspond to any recognized species of this genus. The novel strains shared the same phenotypic characteristics and exhibited 100 % 16S rRNA gene sequence similarity. The nearest phylogenetic relatives of the novel isolates were Lactobacillus satsumensis DSM 16230T (94.2 % 16S rRNA gene sequence similarity), Lactobacillus salivarius JCM 1047 (94.0 %), Lactobacillus nagelii ATCC 700692T (94.0 %) and Lactobacillus saerimneri DSM 16049T (93.8 %). The novel isolates could be distinguished from these species and other related species of the genus Lactobacillus by physiological and biochemical tests. On the basis of these phenotypic, physiological and phylogenetic findings, it is proposed that the new isolates from whales be classified as a novel species of the genus Lactobacillus, Lactobacillus ceti sp. nov. The type strain is 142-2T (=CECT 7185T=CCUG 53626T).