In controlled experiments on damping-off disease, in which small populations of garden cress seedlings (Lepidium sativum L.) were inoculated with Pythium irregulare Buisman, it was found that varying the planting density of the host population had a similar effect on the frequency of primary infection foci to that produced by varying the density of applied inoculum. In both cases, at low densities the number of primary infection foci was proportional to density. Linear regression lines fitted to log-log plots of the density-response data also had slopes approximating to unity. The significance of these findings is discussed in relation to disease control in natural communities.