The pathogenicity for baby-mice of 12 strains of echovirus type 9, isolated in primary monkey kidney cells, from liquor specimens of children with meningitis, have been studied. The animals inoculated with the first passage in tissue culture show after a period of 5 days, a flaccid paralysis and died by the 8th day after inoculation. The original liquor specimens did not contain enough virus to cause clinical signs in the inoculated animals, during the 21 days of observation. Histopathologicals studies in the sick animals, show a necrosis of the paravertebral muscles, tongue and diafragm. Inoculated animals wich did not develop paralysis during the observation period, show light miositis without necrosis of the muscle tissue.