Vapor bubble nucleation on a microrough surface wetted by a volatile, incompressible liquid has been analyzed. The work of formation of a critical nucleus in a surface cavity has been evaluated. Concave sites of a surface with negative curvature can decrease the height of the activation barrier for the formation of a critical nucleus. The average density of the nucleation sites has been evaluated for a surface whose (small) deviation from a plane is specified by a Gaussian random function.