Drastic alterations in the habitats of Schistosoma japonicum in the enzootic area of Taiwan practically eliminated the parasite in the focus studied. Observations over a period of several years allowed a comparison of the status of host-parasite relationships before and after interruption of the parasite cycle by natural causes. Although excessive flooding reduced the snail (Oncomelania formosana) population and the parasite infection in definitive hosts (lower vertebrates) to near nil, Oncomelania populations returned to near normal after approximately 1 year, followed by appearance of infection in rodents. An unusual compensation in biological productivity seemed to allow for rapid reestablishment of the schistosome cycle after abnormal circumstances had placed the parasite-snail-definitive host associations in jeopardy. The inability of the Taiwan strain of Schistosoma japonicum to infect man has been a biological curiosity for years, especially since the island is geographically located near the center of a broad arc of the Orient in which schistosomiasis is a disease of great concern. Confinement to a few small areas, plus a number of other biological factors, separate the species on Taiwan from a morphologically similar parasite designated by the same name in Japan, mainland China, the Philippines, and other areas of Southeast Asia. Continuation of the cycle under adverse conditions has added to the uniqueness of the schistosome situation. S. japonicum continues to propagate itself in spite of very low biotic potentials and other uncertain circumstances relating to zoophilic schistosomiasis on the island. During the past decade Dr. H. F. Hsii and associates (1952, 1962) have elucidated the various peculiarities which differentiate the Taiwan species from others in the Far East. Their early field observations revealed the low rates of infection in Oncomelania formosana in most of the localities studied in Pu-yen Hsiang (township) near Chang-hua. Subsequent observations over a Received for publication 29 March 1965. * Research task MR. 005.09-1601.3, Bureau of Medicine and Surgery, Navy Department. This study was supported in part by funding under Public Law 480, Section 104 (c). The opinion and assertions herein are those of the author and are not to be construed as official or reflecting the views of the Navy Department or Naval Service at large. period of several years in the same area by the present author have similarly indicated rates of infection so low in the snail and lower mammalian hosts that continuation of the cycle appears all but impossible. Factors such as the seasonal reduction of infection rates in mollusks during the winter months, unpredictable drying or man-imposed alterations of snail habitats for parts of the year, intensive rodent control campaigns which eliminate a number of the infected vertebrate hosts, etc., in the enzootic areas, add to the precariousness of host-parasite associations. Hairston (1962) has presented the problems relating to population ecology and has graphically illustrated the mathematical complexities to be surmounted for maintenance of the schistosome cycle in the Philippines, where conditions, though regulated by a multitude of natural variables, probably are more favorable for parasite survival than in Taiwan. During the course of field studies, a natural disaster altered the majority of schistosome habitats and practically eliminated the infection on the west-central coastal plain of Taiwan. For approximately 1 week the S. japonicum habitats were covered as a result of flooding after excessive rains. This occurred in August (1959), the season at which parasite transmission potentials are at their height. Biological activities relating to the parasite cycle within habitats were greatly reduced and temporarily suspended as the rich, well-cultivated and irrigated, alluvial plain of Pu-yen was submerged in 2 to 6 ft of fast-flowing