Root rot of alfalfa (Medicago sativa L.) caused by Aphanomyces euteiches Drechs. is controlled by breeding for resistance. Resistant plants are selected for breeding through disease screens, which use either zoospore or comminuted mycelial inoculum. In the present study, a comparison was made between these two sources of inoculum. Concentrations of 800-900 zoospores per ml and half-plates of mycelium grown on Potato Dextrose Agar blended in 1 liter of water were used. Seedlings were rated using a Disease Severity Index. No significant differences in disease severity occurred in seedlings when inoculated with zoospores versus comminuted mycelium. A large amount of variation between different isolates in regard to zoospore production was observed. Some isolates of the pathogen never produced more than a few zoospores while some individual isolates were variable, and some isolates reliably produced large numbers of zoospores. Production of zoospores increased in both number and reliability as the temperature of incubation was lowered from 24 to 16 C. Significant increase in time and material input for zoospore production versus mycelial inoculum was noted. A temperature effect on disease expression in the disease screen also occurred. It was concluded that comminuted mycelial inoculum was an acceptable inoculum source for the purpose of Aphanomyces root rot disease screens for the selection of resistant plants and that temperature has a profound impact on the system.