A 6-year-old boy was anesthetized with halothane-N₂O-O₂ for anal fistulectomy. During induction of anesthesia, trismus developed soon after intravenous injection of S.C,C. and persisted for several minutes, associated with increased serum creatinine phosphokinase (C.P.K.) levels and elevated body temperature only up to 37.8℃ (without hyperkalemia & myoglobinuria). The operation was postponed and uneventfully performed 4 days later with a normal limit of serum C.P.K. levels, under premedication by intramuscular injection of Valium(diazepam) and induction of anesthesia with halothane-O₂ intravenous injection of Valium for intubation, without any muscular rigidity or increased serum C.P.K. levels. Therefore the authors think there is a difference between this case and malignant hyperpyrexia following anesthesia, and also that the boy had a hyposensitivity predisposition to S.C.C. rather than halothane. Without accurate investigation for predisposition of muscular rigidity, We recommended Valium injection for premedication and induction of anesthesia for prevention of the muscular rigidity.