In two glasshouse and three phytotron experiments, clubroot-susceptible cabbage (Brassica oleracea varcapitata) cv Septa was inoculated with clubroot resting spores at inoculum densities ranging from 0 to 2·107 spores-plant−1. At densities of 105 spores·plant−1 and higher all plants developed clubroot symptoms, except in one glasshouse experiment conducted in winter. The proportion of plants developing symptoms plotted against inoculum density showed a sigmoid curve. Although the shape of the curve was similar in all experiments, the inoculum densities required to induce 50% disease incidence varied from 103 to 105 resting spores·plant−1. The data of all five experiments could be well described by a generalized one-hit model which involves variation between plants with regard to the probability of infection.