A programme to control MD by genetic selection was tested by selecting breeding sires and dams whose progeny were found to have above average resistance to MD following inoculation at day old with infectious material. Under conditions of natural exposure, Australorp pullets from parents selected for resistance had a lower incidence of MD lesions than an unselected Australorp group. In experimentally challenged Australorp chickens the incidence of MD was significantly lower in progeny derived from mating resistant Australorp sires with unselected dams than in progeny from an unselected Australorp line. Under conditions of experimental and natural exposure to MD, crossbred chickens derived from resistant White Leghorn sires and Australorp dams had a lower incidence of MD lesions than the chickens derived from susceptible White Leghorn series and unselected Australorp dams.