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Does adaptation to stress entail fitness costs in optimal environments? Antagonistic pleiotropy sometimes causes trade-offs in fitness across different environments in which the agent of stress is a pathogen (Lenski 1988) or a novel diet (Pashley 1988; Via 1991). It is commonly believed that evolution of resistance to insecticides also involves such trade-offs. Alleles conferring resistance are rare before exposure to insecticide and have been presumed to exert negative effects on fitness in the absence of insecticide (Crow 1957; Uyenoyama 1986; Hoffman and Parsons 1991). However, fitness costs associated with resistance to conventional synthetic insecticides appear to vary considerably among species and insecticides (Roush and McKenzie 1987; Roush and Daly 1990; Denholm and Rowland 1992). Moreover, little is known about fitness costs of resistance to insecticidal proteins from Bacillus thuringiensis, a common soil bacterium (McGaughey and Whalon 1992; Tabashnik 1994). Toxins from B. thuringiensis kill susceptible insects by binding to and disrupting the integrity of the midgut epithelium (Gill et al. 1992). Reduced binding of toxins is a primary mechanism of resistance in the diamondback moth, Plutella xylostella (Ferre et al. 1991), a major pest of cruciferous vegetables (Talekar 1992). The normal function of toxin-binding sites is not known but seems to be important because such sites are found in numerous insects (Feitelson et al. 1992). Several authors have suggested that alteration of toxin-binding sites may interfere with normal physiological functions (Van Rie et al. 1990; MacIntosh et al. 1991). Instability of resistance to B. thuringiensis in the absence of selection