The relationship between inoculum density and mortality or infection was studied for various pepper varieties (Capsicum annuum L.) inoculated with zoospores of two P. capsici isolates. The inoculum concentrations required for 50% mortality (LD 50) varied greatly between pepper varieties and P. capsici isolates: with one isolate, LD 50 was 40 zoospores/ml for a susceptible variety and reached 4,380 to 97,300 zoospores/ml for resistant varieties. For another isolate, LD 50 for the, same varieties ranged from 26 to 800 zoospores/ml. Comparisons between LD 50 and inoculum doses required for 50 % Infection (ID 50) also revealed differences between varieties but not between isolates. After multiple infection correction, regression slopes of infections/inoculum concentration were low for resistant varieties (0.28 to 0.50) but higher for susceptible varieties (0.72 to 0.94), indicating strong competition between spores for infection of resistant plants, but not for infection of susceptible plants. This analysis provided many criteria which can be used to differentiate susceptible from resistant varieties and to evaluate with precision the resistance level of the different resistance genitors used in our breeding program.