SummaryThe pathogenicity and growth rate in vivo were assessed on 27 isolates of Pythium spp. recovered from cavity spot lesions on carrots grown in various parts of northwest France. Polyacrylamide gel electrophoresis of isoesterases was used to identify the Pythium spp. involved.Slow‐growing isolates were more aggressive than fast‐growing ones when inoculated on carrot tap roots. Isoesterase patterns identified the slow‐growing isolates as P. violae and P. sulcatum; P. ultimum and P. intermedium were identified among the less aggressive fast‐growing isolate group, in which some isolates were also classed as P. sylvaticum or P. irregulare, which have similar electrophoretic profiles.The incidence of Pythium spp. associated with the disease in France is discussed in regard to cavity spot in other countries.