Several members of the TRP (for transient receptor potential) family of ion channels act as physiological temperature sensors in mammals1,2,3,4,5,6, but it is not known whether the invertebrate TRP subfamilies that are found in the fruitfly Drosophila and the roundworm Caenorhabditis elegans can be directly activated by temperature. Here we show that the Drosophila orthologue of ANKTM1, which is a cold-activated ion channel in mammals, responds to a warming rather than a cooling stimulus. The thermosensing function of these channels is therefore evolutionarily conserved, and they show a surprising flexibility in their response to different temperature ranges.