Miracidia of Puerto Rican S. mansoni penetrated T. havanensis of Baton Rouge, La., as readily as they did the Puerto Rican A. glabratus. In T. havanensis, but not in A. glabratus, the majority of the larvae were encapsulated by host tissue and destroyed within 15 days. Encapsulated larvae and developing sporocysts were observed in the same tissues of T. havanensis. One to 9 (total 28) normally developing sporocysts were observed in 14 of 58 T. havanensis, each of which had been exposed to 100 miracidia.