Ruminococcus gnavus , a Gram-positive anaerobic coccus, is a common constituent of the human gut microbiota but rarely causes any disease in humans. Herein, we report a case of R. gnavus bacteraemia in an immunocompromised 73-year-old man with sigmoid colon perforation. R. gnavus is usually reported as Gram-positive diplococci or short chains on Gram staining; however, in our patient, a blood isolate showed Gram-positive cocci in long chains, and organisms from an anaerobic subculture showed morphological diversity. This case provides insight into the morphological diversity of R. gnavus , which might help with the recognition of these bacteria in the preliminary identification stage on Gram staining.