Asymmetry of east-west-trending valleys is a prominent landscape feature in the Salinas Valley and Gabilan Mesa area of the central California Coast Ranges. North-facing side slopes in these valleys are significantly steeper, less dissected and more heavily vegetated than south-facing side slopes. This systematic asymmetry persists throughout terrains of widely varying lithology, structural style, and tectonic history, and it is most strongly developed in areas that are underlain by horizontal to very gently dipping semiconsolidated sedimentary rocks. Asymmetric distribution of fill terraces and beheaded streams and close correlation between valley side-slope angle and valley gradient demonstrate that preferential lateral stream erosion has played a dominant role in the development of this valley asymmetry. For most of the valleys studied, this preferential lateral erosion is not related to tectonic movement; it is caused mainly by the asymmetric operation of slope-wasting processes arising from microclimatic differences on opposing valley side slopes.