Peroxidase activity was measured in brown leaf spot pathogen (Myrothecium roridum) inoculated potted mulberry (Morus alba) during pre-symptomatic to various symptom development stages and compared with corresponding healthy leaf tissues. The enzyme showed a pH optimum of 7.0 and the activity was linearly increased up to 15 min of incubation. The peroxidase had a broad substrate specificity and the rates of oxidation were in the rank of pyrogallol> guaiacol> ascorbate at pH 7.0. Catechol at 10 mM inhibited 89% of guaiacol-peroxidase and 76% pyrogallol-peroxidase activities, indicated higher non-specific peroxidation in pyrogallol dependent assay system in mulberry than guaiacol. The optimum requirement for the guaiacol dependent assay was 0.2 ml (${approx}40-60{mu}g$ equivalent of protein) of crude enzyme source. Excepting the 8th leaf from the apex, the peroxidase activity did not vary appreciably in different leaf positions. In pre-symptomatic phases, an initial (1 to 5 min) rise of peroxidase activity was noticed in inoculated leaves, and then maintained a plateau up to 300 min. In contrary, non-infected tissue showed a slightly increased trend of enzyme level up to 420 min. In infected tissue, a sharp transient increase (3.1 fold) of peroxidase activity appeared between 300 - 420 min post infections. Afterwards, significantly different but steady maintenance of enzyme levels were observed in two treatments. On the other hand, during symptom development, a sharp increase in peroxidase activity was noticed up to 4th grade of lesion appearance (25.1 % to 50% of leaf area infection), and then declined slightly. However, in non-infected but same age healthy leaves, such huge fluctuations of enzyme level did not apparent. A high positive correlation $(R^2=0.92)$ between peroxidase activity and leaf spot development grades was also marked. The result implies that pre-symptomatic burst (between 1 - 5 and 300 - 420 min) and subsequent increased trend of guaiacol peroxidase activity may require for the symptomatic manifestation of Myrothecium leaf spot in mulberry.