In lemon fruit inoculated with Phytophthora citrophthora, a continuous increase in H2O2 during the incubation and rot development periods was found at the infection site; seven to ten days after inoculation of the fungus, the mycelium and fruit cells were dead. The suggestion is made that the increase in H2O2 may be related to the decrease in catalase activity and be responsible for the death of the fungus and fruit cells. The effect of different concentrations of peroxide on the fungus in-vitro was tested. © 1976 Dr. W. Junk bv - Publishers.