Summary: Complex shapes bounded by free form surfaces are designed and modified by specifying the parameters of their mathematical descriptions. Although most graphics and design systems provide high level tools for free form shape modifications, they are not intuitive and suitable for a creative designer or a stylist. In the present paper a technique is proposed for the modification of the shape of a parametric B-spline surface through embedding a set of predefined characteristic curves. Shape modification is performed by an evolutionary algorithm. The evolutionary algorithm minimizes the distance between the curve and the surface, while maintaining the global shape and smoothness of the original surface. Several experimental results are presented.