1. A distortion of the shell aperture of Biomphalaria glabrata infected with Schistosoma mansoni reported in earlier work was found to be due to a reversal from ventral to dorsal of the deflection found in uninfected snails.
2. The most sensitive way of measuring this distortion was to record the height of the dorsal deflection. The difference in this figure from uninfected and infected snails of the same diameter is termed the relative distortion.
3. A longitudinal study showed that the relative distortion first appeared at about the time when cercarial shedding started, and that it increased in size linearly as the infection progressed.
4. Temperatures between 20 and 30°C. controlled the length of the prepatent period and thus the first appearance of the relative distortion, but they did not significantly alter its subsequent rate of development.
5. Histological changes were observed in the mantle collar in the region where the relative distortion was most pronounced.
6. Possible mechanisms causing the development of the distortion are discussed.
7. Relative distortions were observed in some naturally infected field snails and the laboratory results were used to estimate how long these snails had been shedding cercariae.
8. Naturally infected St. Lucian snails appeared to have shed cercariae for an average of less than three weeks in the field.
9. A survival curve was drawn up for one collection of field snails, based on the frequency among them of relative distortions of different sizes. Another curve was obtained from an independent technique. The similarity of the two curves gives confidence in both methods.
10. The findings in St. Lucia are discussed in relation to similar studies elsewhere in the world.