We investigate a noncanonical agreement pattern in American English in which a fronted wh-phrase appears to control agreement on an inflected auxiliary, as in Which flowers are the gardener planting? (Kimball & Aissen 1971). We explore this phenomenon with five acceptability-judgment experiments and interpret the resulting data with the aid of a quantitative model of the judgment process. Our study suggests that fronted wh-phrases interfere with agreement primarily as a function of their linear and structural position, and that this effect is not significantly modulated by overt case or thematic cues in off-line judgments. We suggest that our findings support a model of agreement processing in which syntactic phrases compete to control agreement on the basis of their structural and linear position with respect to the inflected verb.