Three cultivars of cabbage (Brassica oleracea L. var. capitata) that differed in reaction to clubroot caused by Plasmodiophora brassicae were identified in field trials. Cultivar Bronco was susceptible to pathotype 6 (all plants severely clubbed), ‘Kilaherb’ was resistant (no plants with symptoms), and ‘B-2819’ was intermediate (small clubs). The development of P. brassicae in the roots of these cultivars was then assessed under controlled conditions at 28 days after inoculation using root sectioning, staining with methylene blue, and bright field microscopy. Plasmodia of P. brassicae were present in the root tissue of each cultivar. There was extensive cortical colonization (26 %) and some resting spore formation in ‘Bronco’, and lower colonization (9 %) and only trace levels of resting spore formation in ‘B-2819’. In the clubroot-resistant ‘Kilaherb’, root colonization was similar (7 %) to the intermediate ‘B-2819’, but no resting spores formed and no clubbing symptoms developed. This pattern of response differed substantially from that observed previously in commercial cultivars of spring canola (B. napus), and may indicate that the mechanism of resistance in ‘Kilaherb’ is different from that currently available in canola. Also, the intermediate phenotype of ‘B-2819’ may represent a quantitative resistance reaction that may also differ from that in canola.