Structural analogy is the assumption that structural differences between the levels and planes (levels with different alphabets) of linguistic representation are severely constrained: within the limits imposed by the character of inter-level relationships and by differences in alphabet, we expect the same properties to recur on different levels and planes. The implementation of the case grammar hypothesis (that semantic roles are syntactically basic) is explored here in terms of this assumption; and the hypothesis is shown to interact with other constraints imposed by the structural analogy assumption to provide a restrictive theory of syntactic structure and the lexicon-syntax interface.