Endotrypanum schaudinni, a haemoflagellate of sloths, has a very wide geographical distribution in America, stretching from the Amazonian Basin in the South to Costa Rica in the North. It has been shown that the infection is limited to animals which live in forest regions1. Sloths of the genera Bradypus (three-toed sloths) and Choloepus (two-toed sloths) from the Atlantic side of the Isthmus of Panama were examined, and 54 per cent of the two-toed sloths were found infected with E. schaudinni. This area was, therefore, highly endemic for endotry-paniasis, suggesting that a highly efficient transmission was occurring at the time. No infection was found, however, in 11 three-toed sloths, but this genus apparently seldom enters the endemic rain-forest habitat.