INTRODUCTION: Primary arthritis is a rare form of meningococccal disease. It occurs as an isolated acute purulent arthritis without meningitis, and presence of Neisseria meningitidis in articular fluid. PATIENT AND OBSERVATION: We report the case of an infant aged 45 days without any particular medical history presenting isolated primitive monoarthritis of the left knee to Neisseria meningitidis group B unhurt of meningeal infringement, at Children's Abderrahim El Harouchi hospital of Casablanca. The examination in the admission showed a feverish infant, with presence of a painful and inflammatory effusion of the left knee. The neurological examination was normal and did not reveal signs of meningeal infringement. The articular draining resembled a purulent liquid whose cytobacteriogical examination showed the presence of neutrophils in heap, the culture revealed the presence of Neisseria meningitidis. The serogroup allowed to show that it was about the serogroup B. This result was confirmed by molecular biology (PCR). The study of the sensitivity in antibiotics by E-test revealed an origin sensitive to the penicillin G and to the 3rd-generation cephalosporin. An evacuation of the articular effusion was performed and an antibiotic treatment with ampicillin-sulbactam and of gentamicin was administred, allowing a favorable evolution within a few days. CONCLUSIONS: The diagnosis of primitive arthritis with meningococcus without meningeal infringement is infrequent, but probably underestimated because of the rarity of the practice of the articular punctions and the blood cultures in front of these situations. The diagnosis and the early treatment of this pathology allow a cure without long term-side effects.