Transformation efficiency in competent variants of theNeisseria meningitidisStrain M1 increased more than three‐fold after exposure of the recipient cells to approximately 35 atmospheres osmotic pressure for 20 min. Growth of the recipient cells at increased osmotic pressure constantly reduced the transformation efficiency. Exposure of the cells to hypotonic conditions reduced transformability significantly and attempts to restore transformability by means of supernatant solutions from competent cultures were unsuccessful. Incompetent variants of the Strain M1 could not be rendered competent by exposure to increased tonicity. The findings have been discussed in relation to the effects of tonicity variation onN. meningitidiscompetence variant