The subcellular distribution of a basic class IV chitinase (Ch4) in Cercospora beticola-infected sugar beets has been studied by immunohistological analysis. Ch4 was primarily deposited in extracellular globuli proximal to the necrosis. High levels of Ch4 were observed in the vicinity of the necrotic lesions, low levels were found at distant sites from the necrosis. This plant-pathogen interaction thus gives rise to a local induction of Ch4, whereas no apparent systemic induction of Ch4 was found. After infection with Cercospora, a tolerant sugar beet cultivar accumulated Ch4 faster than a susceptible one, and the protein was spatially distributed over a comparatively larger area in the tolerant variety. In the tolerant beet cultivar symptoms appear a few days later. and the number of necroses are reduced and smaler than in the sensitive cultivar. In uninfected sugar beet leaves, Ch4 is present in low amounts in epidermis, stomata and xylem cells. A specific autofluorescence and a high endogenous beta-glucuronidase activity in Cercospora were used to demonstrate that this pathogen did not penetrate the tissue beyond the necrosis.