It is usually believed that snakes of the family Colubridae, when biting, must hold on at least several seconds in order to inject doses of toxic Duvernoy's glands secretion that are sufficient to cause envenomation in humans, and that quick defensive bites would not result in envenomation. However, some colubrid species are known to have caused envenomation by quick bites. Two species of the South American genus Philodryas can now be added to this kind of colubrids. A 13-year-old victim of a bite by P. olfersii latirostris COPE, 1862 developed local swelling, enlargement of axillary lymph nodes, joint stiffness in the upper extremities and the knees, and ecchymoses on the knees. This case demonstrates that P. olfersii is capable of inflicting systemic human envenomation with only a quick defensive bite. The predominant symptoms following a quick bite by P. chamissonis (WlEGMANN, 1834) inflicted on the same person 4.5 years later, were numbness and coldness of the bitten arm, pain, slight swelling, and joint stiffness of hand and wrist. Previous records, and the rapid onset of envenomation after quick defensive bites described herein, suggest P. olfersii and P. chamissonis to be regarded as venomous snakes in a clinical sense.