In its natural environment, Drosophila melanogaster feeds on yeasts that grow on sugar-rich substrates such as fermenting fruit. Fruits, however, also harbor toxic microbes, and flies need to distinguish those microbes that are safe and nutritious from the harmful ones. In this issue, Stensmyr et al. (pp. 1345-1357) demonstrate that flies detect toxic molds by sensing a volatile compound called geosmin, which exclusively triggers a dedicated signaling pathway in the flies' olfactory system. This circuit, upon activation, causes innate aversion and also prevents egg laying and feeding. Cover concept by Rakel and Marcus Stensmyr. Clay modeling and photo by Marcus Stensmyr.