Caterpillars of Maculinea rebeli have two growth strategies for living underground as social parasites of Myrmica ant colonies. Laboratory experiments and field data show that 25% of caterpillars live ten months with ants before pupating, whereas 75% grow slowly, parasitizing their hosts for 22 months. Both types of caterpillar form apparently identical similar–sized pupae. This may be the first description in the animal kingdom of polymorphic growth rates spanning different years within the same population, yet without resulting (as in salmonid fish) in two morphologically distinct adult types with obvious differences in behaviour. We suggest that a balanced polymorphism has evolved in M. rebeli growth rates, representing the most efficient way of exploiting the limited, yet steady, daily supply of food available to cuckoo–feeding parasites of long–lived ant societies. Bet–hedging benefits would also accrue to adult butterflies producing a mixture of annual and biennial offspring. Despite ergonomic and other benefits, partial biennialism is unlikely to evolve unless slow–growing individuals have enhanced survival and can remain attached to their mobile hosts. We show that caterpillars become so closely protected by, and integrated with, their host colonies that slow growers experience no greater mortality over two years than fast growers over one, and are transported in preference to the ants' own larvae when the host colony moves nest site.