Data from the pre-epidemic period and from the present epidemic of meningococcal meningoencephalitis in the Great Rio de Janeiro area are presented. A thousand suspected cases were bacteriologically studied after lumbar spinal puncture at the Hospital Estadual São Sebastião, in Rio de Janeiro. Aiming at a rapid and simple diagnostic procedure to be applied to spinal fluids errors and hazards of some techniques, based on stained microscopic slides and limited choice of culture media, are discussed. Isolation of etiologic agents varied from three per cent in specimens with less than 10 white cells/mm³ to 72 per cent with fluids containing more than a thousand cells/mm³. The distribution of 356 isolated strains correspended to: Neisseria meningitidis (281); Haemophilus sp. (22); Enterobacteriaceae (15); pneumococci (26); Gram-negative oxidative rods (3); beta hemolytic streptococci (1) and enterococci (1). Serologically the meningococci corresponded to 51 per cent group A, 2 per cent group B and 14 per cent group C strains. The remaining strains were not grouped with the mentioned available diagnostic sera or demonstrated spontaneous agglutination. Very few cases of resistance to antibioties were found by using the diffusion sensitivity tests with discs of the common antimeningococcal drugs. Sulphadiazine resistence was frequent, however, particularly within the group C meningococcal isolations, when the dilution tests in solid media, according with the Food and Drug Administration (U.S.AJ instructions were employed.