An isolate of Fusarium oxysporum (MT-6) was obtained from pasture soil in New Zealand in 1987. The isolate was grown on rice and fed to rats in a ratio of 50% and 10% of a complete diet. All rats died within 16 hr and showed mild intestinal and thymic hemorrhage. The major identifiable toxin was moniliformin found at a concentration of 9 mg/g in the rice culture. Purified crystals of moniliformin fed to rats at concentrations of 8, 5 and 3 mg/g of rat diet caused hemorrhaging in the intestines and death of 3 of 3 rats within 16 hr. Dietary concentrations of 2.5 and 2 mg/g of moniliformin killed 2 of 3 rats and concentrations of 1.5 and 1 mg/g killed 1 of 3 rats. No lesions were observed in 3 groups of 3 rats fed a diet containing 0.75, 0.5 or 0.25 mg/g of moniliformin. Intragastric intubation of moniliformin caused hemorrhage of the small intestine and death in 5 of 5 rats at each concentration of 100, 80, 60 and 40 mg/kg body weight and death of 4 of 5 rats at 20 mg/kg. No observable toxic effects were observed in groups of 5 rats each receiving 10, 5 and 2.5 mg of moniliformin/kg body weight.