Trade-offs, or negative genetic correlations, in fitness on different hosts have been hypothesized to restrict evolutionary expansion of host range in phytophagous arthropods. I used a population of the polyphagous mite Tetranychus urticae that had been selected for adaptation to tomato plants to determine whether tradeoffs between fitness on tomato and fitness on lima bean occur in this species. Two lines established from the tomato-adapted population and reared on lima bean showed significant declines in ability to survive on tomato after more than 10 generations on bean. The most likely explanation for this result is that tomato-adapted genotypes were selected against on bean. The tomato-adapted population did not have detectably lower survival or fecundity on bean than a control population kept on bean, however, suggesting that tomato-adapted genotypes were at only a slight disadvantage on bean. This study and a similar previous experiment with T. urticae using cucumber and bean provide the only evide...