The first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the Kettle dome metamorphic core complex, northeastern Washington. The mylonite zone developed at mid-crustal levels and has undergone a minimum of retrogression and cataclasis during uplift. Downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5-km depth as a complex zone of multicyclic reflections. The seismic data also show that the crust beneath the Kettle dome is characterized by abundant, strongly reflective, moderately dipping layers down to at least 18 km. These layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both. Models for the structure and genesis of metamorphic core complexes must take these data into account.