A coccobacillus (plural coccobacilli) is a type of bacterium with a shape intermediate between cocci (spherical bacteria) and bacilli (rod-shaped bacteria). Coccobacilli, then, are very short rods which may be mistaken for cocci. Haemophilus influenzae, Gardnerella vaginalis, and Chlamydia trachomatis are coccobacilli. Aggregatibacter actinomycetemcomitans is a Gram-negative coccobacillus prevalent in subgingival plaques. Acinetobacter strains may grow on solid media as coccobacilli. Bordetella pertussis is a Gram-negative coccobacillus responsible for causing whooping cough.
Coxiella burnetti is also a coccobacillus. Bacteria from the Brucella genus are medically important coccobacilli that cause brucellosis. Haemophilus ducreyi, another medically important Gram-negative coccobacillus, is observed in sexually transmitted disease, chancroid, of Third World countries.