Interspecific mutualisms are regarded as having evolved from antagonistic or commensalistic interactions, with most mutualisms remaining facultative but some having coevolved into obligate reciprocal dependency(1-4). Underlying mutualism is an intrinsic conflict between the parties, in that each is under selection for increased exploitation of the other(3-7). Theoretical models suggest that this conflict is a source of evolutionary instability, and that evolution of ’cheating’ by one party may lead to reciprocal extinction(4,6,7). Here we present phylogenetic evidence for reversal of an obligate mutualism : within the yucca moth complex, distinct cheater species derived from obligate pollinators inflict a heavy cost on their yucca hosts by laying their eggs but not pollinating the yucca. Phylogenetic data show the cheaters to have existed for a long time. Coexisting pollinators and cheaters are not sister taxa, supporting predictions that evolution of cheating within a single pollinator is evolutionarily unstable. Several lines of evidence support a hypothesis that host shifts preceded the reversal of obligate mutualism. Host or partner shifts is a mechanism that can provide a route of evolutionary escape among obligate mutualists in general.