Analogy is often viewed as a partial similarity match between domains. But not all partial similarities qualify as analogy: There must be some selection of which commonalities count. Three experiments tested a particular selection constraint in analogical mapping, namely, systematicity. That is, we tested whether a given predicate is more likely to figure in the interpretation of and prediction from an analogy if the predicate participates in a common system of relations. In Experiment 1, subjects judged two matches to be included in an analogy: on isolated match, and a match embedded in a larger matching system. Subjects preferred the embedded match. In Experiments 2 and 3, subjects made analogical predictions about a target domain. Subjects predicted information that followed from a causal system that matched the base domain, rather than information that was equally plausible, but that created an isolated match with the base. Results support Gentner's (1983, 1989) structure-mapping theory in that analogical mapping concerns systems and not individual predicates, and that attention to shared systematic structure constrains the selection of information to include in an analogy.