This paper describes a case of non-lexical borrowing in the Northwest Amazonian language Resígaro (Arawakan), which has borrowed from the unrelated Bora language entire paradigms of noun class, gender, and number markers, as well as associated bound grammatical roots, while all other morphosyntactic subsystems of Resígaro are virtually unaffected. To account for this case of massive morphological borrowing (and others that have previously been described), this paper proposes the Principle of Morphosyntactic Subsystem Integrity (PMSI), which predicts that in situations where various grammatical morphemes are borrowed, these tend to be morphosyntactically interrelated, rather than being random collections of forms or sets of forms that are best described by well-known borrowability hierarchies, e.g. lexical before grammatical morphemes or derivational before inflectional markers.