1. Transmission of the virus of urban typhus under experimental conditions from rat to rat by the rat flea (X. cheopis) by feeding has been effected. Collateral attempts to transmit the virus of rural typhus by precisely the same procedure failed.
2. Transmission of the virus of urban typhus was also achieved by the inoculation of faeces or crushed tissue of infected fleas into the scarified skin of guinea-pigs.
3. Multiplication of the virus of urban typhus occurs within the rat flea.
4. Infection with the virus of urban typhus is not hereditary in the rat flea.
5. Attempts to transmit the virus of urban or rural typhus by two species of ticks failed. In the case of rural typhus a lessened mortality in the experimental guinea-pigs following test inoculation with passage virus makes it, however, difficult to exclude ticks entirely as a minor factor in the epidemiology of rural typhus.