We describe the modification of a grammar to take advantage of prosodic information provided by a speech recognition system. This initial study is limited to the use of relative duration of phonetic segments in the assignment of syntactic structure, specifically in ruling out alternative parses in otherwise ambiguous sentences. Taking advantage of prosodic information in parsing can make a spoken language system more accurate and more efficient, if prosodicsyntactic mismatches, or unlikely matches, can be pruned. We know of no other work that has succeeded in automatically extracting speech information and using it in a parser to rule out extraneous parses.