This paper reports on an implementation of a multimodal grammar of speech and co-speech gesture within the LKB/PET grammar engineering environment. The implementation extends the English Resource Grammar (ERG, Flickinger (2000)) with HPSG types and rules that capture the form of the linguistic signal, the form of the gestural signal and their relative timing to constrain the meaning of the multimodal action. The grammar yields a single parse tree that integrates the spoken and gestural modality thereby drawing on standard semantic composition techniques to derive the multimodal meaning representation. Using the current machinery, the main challenge for the grammar engineer is the non-linear input: the modalities can overlap temporally. We capture this by identical speech and gesture token edges. Further, the semantic contribution of gestures is encoded by lexical rules transforming a speech phrase into a multimodal entity of conjoined spoken and gestural semantics.