Typical trypanosomes are characterized by a comparatively long, spirally twisted body, 8 to 35 µ, along one side of which is attached an undulating membrane having a cord-like edge that is continued forward as a flagellum. This flagellum arises near the posterior end of the organism in a small granule, called the blepharoplast, which lies near, or may be merged with, a large chromatin mass, called the kinetonu-cleus, because of its control over the motor apparatus. The nuclear apparatus consists of a trophonucleus with an intranuclear centrosome.

Multiplication takes place by longitudinal division. The centrosome first divides, the posterior end then shows signs of cleavage; the undulating membrane is soon seen to double beyond the point of separation, and the division extends anteriorly until connected only by the flagellum, which soon separates.

Life cycle is not well known. Though transmission occurs through the bites of various invertebrates, notably flies, no definite sexual changes have been proved to take place in the intestines of these intermediate hosts. In one variety, "dourine," no intermediate host seems necessary, the infection occurring through coitus.

Many of these organisms have been artificially cultivated.

These bodies are found in the blood in certain diseases of man and the lower animals.