The causative agent of sleeping sickness in humans, Trypanosome brucei brucei, has been shown to exhibit social motility, or coordinated migration, on semi-solid surfaces. Imhof et al. used a series of mutants to investigate when T. b. brucei displays social motility during its developmental cycle in vivo. Within its tstese fly vector T. b. brucei must migrate twice, first during the early procyclic stage from the midgut lumen to the ectoperitrophic space and second, during the late procyclic stage from the ectoperitrophic space into the salivary glands. The authors found that social motility was displayed only during the early procyclic stages, in the first week after uptake by the tsetse fly.