Speciation induced by parasitic or mutualistic symbionts has been suggested for taxa ranging from plants to insects to monkeys. Previous models for symbiont-induced speciation have been based upon hybrid inferiority and selection for reinforcement genes. Taken on their own, however, such models have severe theoretical limitations and little empirical support. Two conditions that may favour symbiont-induced speciation are presented here: (1) interaction norms in which the outcomes of host/symbiont interactions differ between environments and (2) differential coadaptation of host and symbiont populations between environments or along an environmental gradient. Symbiont-induced speciation can be considered as one form of ‘mixed-process coevolution’: reciprocal evolution in which adaptation of a population of one species to a population of a second species (or coadaptation of the populations) causes the population of the second species to become reproductively isolated from other populations.