The ecological functions of many toxins continue to remain unknown for those produced by environmental pathogens. Mycobacterium ulcerans, the causative agent of the neglected tropical disease, Buruli ulcer, produces a cytotoxic macrolide, mycolactone, whose function(s) in the environment remains elusive. Through a series of dual-choice behaviour assays, they show that mycolactone may be an interkingdom cue for the yellow fever mosquito, Aedes aegypti, seeking blood-meals as well as oviposition sites. Results provide novel insight into the evolution between bacteria and potential vectors. While further studies are needed to determine if mycolactone is an actual signal rather than simply a cue, this discovery could serve as a model for determining roles for toxins produced by other environmental pathogens and provide opportunities for developing novel strategies for disease prevention. The relationship between M. ulcerans, mycolactone, and Ae. aegypti further suggests there could be an amplification effect for the spread of pathogens responsible for other diseases, such as yellow fever and dengue.