In current phonological theory, borrowings are assumed to exhibit a fixed set of foreign features which distinguish them from the native vocabulary of a given language. In such a theory, assimilation of the borrowing appears to be a sudden loss of these features. The data presented here indicate that (1) the foreign features are not uniformly distributed over all segments of a given borrowing, and (2) the assimilation of individual features in borrowings to their target phonetic constraints proceeds at different rates. The rate of assimilation is shown to be a function of the general target constraint itself, of the segment class affected by the constraint, and of the syllable on which the constraint operates. It is hypothesized that this rate of assimilation is a measure of the strength or productivity of the rule.