A controlled reading experiment reveals that stress-based linguistic rhythm impinges on syntactic ambiguity resolution in silent and oral reading. The results suggest that, at points of syntactic underspecification, the accruing prosodic representation may affect even the earliest stages of structure building, viz. the analysis of syntactic features of an ambiguous word. Such an effect remains inexplicable in the context of (psycho-)linguistic theories that assume a strictly unidirectional relationship between syntactic and phonological processes, the latter merely interpreting the conditions the syntactic component imposes on it. Here, a performance compatible competence grammar in the framework of Optimal Parsing is presented that is capable of capturing the reading data. The model integrates syntactic parsing and prosodification in reading and predicts that, at points of syntactic indetermination, weak prosodic constraints alone may guide syntactic structure assignment. This suggests a bidirectional relationship between syntax and phonology in grammar and processing while, at the same time, confirming a tight coupling of language production and comprehension.