The timing of seasonal host activity, or host phenology, is an important driver of parasite transmission dynamics and evolution. Despite the vast diversity of parasites in seasonal environments, the impact of phenology on parasite diversity remains relatively understudied. For example, little is known about the selective pressures and environmental conditions that favor a monocyclic strategy (complete a single cycle of infection per season) or a polycyclic strategy (complete multiple cycles). Here, we present a mathematical model that demonstrates that seasonal host activity patterns can generate evolutionary bistability in which two evolutionarily stable strategies (ESSs) are possible. The ESS that a particular system reaches is a function of the virulence strategy initially introduced into the system. The results demonstrate that host phenology can, in theory, maintain diverse parasite strategies among isolated geographic locations.