Six Gram-positive-staining, microaerophilic, non-spore-forming, fructose-6-phosphate phosphoketolase-positive bacterial strains with a peculiar morphology were isolated from faecal samples of baby common marmosets (Callithrix jacchus). Cells of these strains showed a morphology not reported previously for a bifidobacterial species, which resembled a coiled snake, always coiled or ring shaped or forming a ‘Y’ shape. Strains MRM 3/1^T and MRM 4/2 were chosen as representative strains and characterized further. The bacteria utilized a wide range of carbohydrates and produced urease. Glucose was fermented to acetate and lactate. Strain MRM 3/1^T showed a peptidoglycan type unique among members of the genus Bifidobacterium . The DNA base composition was 64.7 mol% G+C. Almost-complete 16S rRNA, hsp60, clpC and rpoB gene sequences were obtained and phylogenetic relationships were determined. Comparative analysis of 16S rRNA gene sequences showed that strains MRM 3/1^T and MRM 4/2 had the highest similarities to Bifidobacterium scardovii DSM 13734^T (94.6 %) and Bifidobacterium stellenboschense DSM 23968^T (94.5 %). Analysis of hsp60 showed that both strains were closely related to B. stellenboschense DSM 23968^T (97.5 % similarity); however, despite this high degree of similarity, our isolates could be distinguished from B. stellenboschense DSM 23968^T by low levels of DNA–DNA relatedness (30.4 % with MRM 3/1^T). Strains MRM 3/1^T and MRM 4/2 were located in an actinobacterial cluster and were more closely related to the genus Bifidobacterium than to other genera in the family Bifidobacteriaceae . On the basis of these results, strains MRM 3/1^T and MRM 4/2 represent a novel species within the genus Bifidobacterium , for which the name Bifidobacterium aesculapii sp. nov. is proposed; the type strain is MRM 3/1^T ( = DSM 26737^T = JCM 18761^T).