Mallards (Anas platyrhynchos platyrhynchos) and white pekin ducks (Anas platyrhynchos domesticus) were infected with duck plague virus and challenged with LD20's of Pasteurella multocida and P. anatipestifer. There was no difference between mortality rates of duck plague-infected ducks and controls, suggesting that these organisms do not act synergistically under the conditions of our experiments. There was a difference of about 500-fold between the LD20 of P. multocida for mallards and that for white pekin ducks, indicating that mallards are much more susceptible to avian cholera than white pekin ducks.