Summary Nine additional species and subspecies of native American planorbids have been exposed to infection with Schistosoma mansoni, with negative results; additional attempts have been made to infect certain species which had been tested earlier. Additional laboratory studies have been conducted with a species of Tropicorbis which has proved susceptible to infection. To date, 9 specimens have been infected; all of these snails came, or were progeny of snails which came, from a lake on the campus of Louisiana State University at Baton Rouge, Louisiana. Subsequent to exposure of 22 wild adult specimens and of 77 laboratory-reared juveniles, 2 and 7 snails, respectively, or 9 percent, shed cercariae of S. mansoni. On the other hand, there are described unsuccessful attempts to infect tropicorbids which, however, appear to be different species and which were collected from two different sites.