Summary The presence of T. ariarii in the hemolymph and in the salivary glands of R. prolixus (both naturally and experimentally infected) is a rather constant feature. Furthermore the ability of R. prolixus to transmit T. ariarii through the bite has been proved. These data seem to point to a cyclical development of T. ariarii in R. prolixus, in which the flagellates are first present in the intestine after the infective meal. Later on, some of the parasites migrate to the hemolymph and from this to the salivary glands. From the glands the trypanosomes can be transmitted—through the bite—into new vertebrates.