Discussion and Summary The origin and time of introduction of Marisa into the San Anton Creek cannot be definitely established. This species is a popular aquarium snail and it is possible that this has facilitated its spread. It is known to be common in Brazil, northern South America and Central America (Baker, 1930). In recent years it has appeared in Cuba (Penalver, 1950) and the method of introduction is not known. The San Anton Creek apparently offered more or less ideal conditions for this snail. This, in addition to the possible lack of natural enemies, led to its almost explosive reproduction. This phenomenon coincided with the disappearance of Australorbis from this habitat but other factors may have played a part. As shown in Table 2, only small numbers of Marisa were present in the last collections at Station 5, therefore, it is possible that the factors that caused this reduction may have also affected the other species. At all other stations, however, large populations of Marisa were found. Observations made on La Palma Pond gave further support to the hypothesis that Marisa was actually the decisive factor that caused the disappearance of Australorbis. It remains to be seen whether Marisa will remain in the ascendancy in San Anton Creek and La Palma Pond, or if stabilization in a new environment will eventually lead to an adjustment whereby both species may survive in the same habitat. Any method that reduces the number of Australorbis may, however, place the population below the threshold necessary for maintaining the transmission of schistosomiasis. Laboratory and field observations on the feeding habits of Marisa indicate that this snail is a voracious feeder. This suggests two factors that may have caused the reduction in the Australorbis population. First, it appeared possible that this snail ingested the eggs of Australorbis that had been laid on the vegetation. Secondly, the presence of a large number of Marisa has somewhat the same effect as a vacuum cleaner and the reduction in population may be due to a serious depletion of the food supply. The findings of Chernin et al., (1955) under laboratory conditions indicate that both of these factors probably play an important role. Australorbis was found to be plentiful in one small tributary of the creek under observation. The reason why Marisa did not invade this small stream may be due to size differences in the two species. The diameter of the adults of Australorbis from this stream is about 15 mm., as compared with about 60 mm. for the other species. The latter found ideal conditions in the main body of the stream but did not extend into the tributary. In many habitats where Australorbis is found in Puerto Rico, the water is often so shallow that the larger snail would not be able to remain submerged. This may be a limiting factor in the distribution of Marisa. As far as is known M. cornuarietis does not serve as a host that produces any of the cercariae of trematodes that would infect man or domestic animals. In our laboratory repeated exposure of this snail to the miracidia of S. mansoni failed to produce infection. Therefore, it is planned to introduce the species into other selected Australorbis habitats, under controlled conditions and where adequate ecological and population studies have been made. Under such conditions it should be possible to determine whether this species would be of use in the biological control of one of the vectors of S. mansoni. The authors are grateful to Mr. Nelson Biaggi, from the Department of Preventive Medicine and Public Health, School of Medicine-School of Tropical Medicine, for his cooperation during this study. Also to Dr. D. B. McMullen, Department of Medical Zoology, Army Medical Service Graduate School, Washington, D. C., for his revision of the manuscript.