The trypanosome to be described in this paper was discovered in blood films taken from the snake Erythrolamprus aesculapii (Duméril and Bibron) of tropical America. For these films I am indebted to Dr Leiper. In addition to the trypanosome there was present in the blood a haemogregarine. Though haemogregarines are very common in snakes, especially in the Tropics, where nearly every snake examined is found to harbour these parasites, the reverse is the case with trypanosomes. Several observers have recorded the presence of trypanosomes in snakes but hitherto no one has given an accurate description of one of trypanosomes of the whole group of reptiles is very limited when campared with other groups of vertebrata. On this account it seems of interest to place on record the characters of this trypanosome as it appears in the blood films mentioned above.