Switch reference, in which certain clauses contain a signal indicating whether that clause has the same or a different subject, is examined in Mbya Guarani. It is found that most cases can be covered by a grammatical rule stated in terms of the grammatical subjects of the two clauses involved, yielding "same subject" and "different subject" markers. In sentences where this subject-related dichotomy is complex, switch-reference marking can instead be used to indicate facts of a semantic or pragmatic nature, such as whether the clauses have the same agent/topic or the same semantic type. These are considered marked uses of Mbya switch reference. Certain aspects of these uses can be described by rules similar to grammatical ones, but the description is essentially external, depending on factors outside the formal system. Mbya switch reference is therefore one example of linguistic phenomena conditioned by grammatical features and amenable to description by grammatical rule, but only in the unmarked case. In marked modes of use, they are conditioned by extragrammatical factors. More specifically, Mbya switch reference is a "fair-weather phenomenon" whose marked uses are triggered by complexities arising with its unmarked use. Further questions arising from this analysis are discussed briefly. (MSE)