High resistance of Indonesian thin tail (ITT) sheep against Fasciola gigantica has been confirmed. Naive ITT sheep had only 17% of the number of mature parasites collected from control St. Croix sheep. In contrast, the level of resistance of ITT sheep against F. hepatica was the same as that of the low resistance Merino breed after both primary and secondary infections. It is suggested that resistance of ITT sheep against F. gigantica was manifested in two phases. The major phase appeared to be specific to the ITT sheep : F. gigantica relationship, acting against immature parasites in both naive and previously exposed hosts and could be innate or acquired. The second phase appeared to be specific to F. gigantica, and was acquired, because it killed young adult parasites only after secondary infection. F. gigantica from ITT sheep were heavier than those from the St. Croix sheep, possible because feeding was impaired by more extensive liver damage in the latter. Approximately 45 F. gigantica killed 25-kg sheep before and during migration from the liver parenchyma into the bile ducts. Death was caused by haemorrhages into the liver parenchyma, bile ducts and peritoneum.