Inducible defensive traits against herbivores or predators are widespread in plants and animals. Theory predicts that defended morphs have greater fitness in the presence of predators, but lower fitness than undefended morphs in the absence of predators. If such costs did not exist, then a constitutively defended morph would be favored by natural selection; yet, evidence for such costs has been elusive. Our current work reveals a significant cost to inducible defenses. Using the waterflea (Daphnia) model system, we show that induced defended morphs are significantly more vulnerable to infection by a virulent yeast parasite than undefended morphs. In two independent experiments, the proportion of successful infections and the number of parasite spores were higher among defended versus undefended Daphnia. Thus, by demonstrating a previously unknown and environmentally relevant cost to inducible defenses, this study enhances our understanding of adaptive phenotypic plasticity and its evolution.