Many organisms living in heterogeneous environments alter behaviors or morphology when developing in the presence of predators and subsequently incur associated sublethal fitness costs. Larvae of the mayfly Callibaetis ferrugineus hageni develop in beaver ponds with or without trout predators. We examined the potential sublethal fitness costs of developing in the presence of trout predators by comparing patterns of timing and size at emergence from ponds over two years, before and after manipulating trout densities. In addition, the behavior, timing, and size at emergence of larvae reared in mesocosms were compared between treatments with and without trout chemical cues. Timing of mayfly emergence and adult size was not affected by the manipulation of trout in the field, or by the presence of trout chemical cues in mesocosms. Observations of mayflies in mesocosms provided no evidence that predator cues induced antipredator behaviors, such as reduced activity or increased crypsis. Surprisingly, late instar larvae swam more frequently in the presence of trout cues during the middle of the day, a behavior that could increase their vulnerability to visually feeding predators. Thus, larvae did not exhibit any traits expected to increase survival in the presence of trout cues. The apparently maladaptive responses to trout may result from phylogenetic inertia or conflicting selection pressures encountered when developing in fishless habitats. The evolution of plasticity to trout in Callibaetis may be inhibited by frequent dispersal from fishless source populations.