The host range of Pythium sulcatum, the cause of cavity spot disease of carro ts in most carrot-growing regions of Australia, was determined by growing seedlings in infested soil in the field. P. sulcatum was isolated from roots of carrots and other members of the family Apiaceae, but not from vegetables from other plant families and grasses. Control by rotation with a non-host, broccoli, was attempted on a badly infested site. The incidence and severity of seedling infection by P. sulcatum was significantly reduced when carrots followed one, two or three broccoli crops. Reduced seedling infection was associated with decreased forking and increased root length at harvest, resulting in an increase in the proportion of export market quality carrots. There was a decrease in the incidence and severity of cavity spot in two of the three plantings where carrots followed broccoli. Oospores of P. Sulcatum are able to survive for at least 21 months in the absence of a host.