Although much is known about factors which determine infection rates of salivarian trypanosomes (subgenera Nannomonas, Duttonella and Tryanozoon) in the tsetse fly Glossina, it is not clear why infection rates of Trypanozoon are high in mammalian hosts but low in wild-caught Glossina and why trypanosomiasis occurs where Glossina is not readily detectable. We report here that the feeding behaviour of trypanosome-infected Glossina differed from that of uninfected control flies. Infected flies probed more frequently and fed more voraciously. We describe a specific relationship between trypanosomes and the mechanoreceptors responsible for detecting the rate of blood flow, and show how infection affects that rate in the labrum. We suggest that the observed differences in feeding behaviour result from impaired function of the labral mechanoreceptors in infected Glossina.