1. Guinea-pigs, naturally uninfected by sarcosporidia, were infected by feeding them with rat's muscle that was naturally infected by Sarcocystis muris, and by ripe mobile sporozoites from the same source. The infection was not visible grossly, but was detected upon very careful search through many sections of muscle.
2. Sarcosporidia were not found in the guinea-pigs until after an interval of 164 days from the first feeding, or 152 days after the most favorable feeding, when many teased-out mobile sporozoites were fed.
3. The prolonged period of incubation or latency, and the greater time required for infection of guinea-pigs here over that required by Negri (7) in Pavia, may be related to the fact that the experiment here was conducted in the tropics where the guinea-pig is a native.
4. Sarcosporidia of this apparently abortive type in unusual hosts cannot be specifically identified until their derivation and host relationships have been determined.
5. Morphologically, the guinea-pig sarcosporidia derived from Sarcocystis muris are identical with those found by the writer in the biceps of a Barbadian negro, and both probably represent abortive or aberrant forms.