S U M M A R YCallus cultures established from various sugar beet varieties on complex and chemically defined media were infected with the sugar beet downy mildew fungus Peronospora farinosa under sterile conditions. The host callus and the parasite grew in complete balance in culture, and new infected cultures could be established by transferring explants of infected callus to fresh medium. Peronospora farinosa in callus culture produced normal intercellular hyphae with digitate haustoria and conidiophores with conidia, and remained pathogenic to sugar beet plants for nearly 2 years. The fungus grew for short distances away from infected calluses on the surface of an agar culture medium, but did not grow axenically when connections with these calluses were severed. Resistance to P. farinosa was not expressed normally in sugar beet calluses.