A leakproof plastic reclosable fastener or zipper with slider particularly suited for thermoplastic bags. The zipper is formed by reclosable interlocking rib and groove profile elements on respective strips and the strips each include profiled track structure extending along the length thereof parallel to the rib and groove elements. A straddling slider is positioned on the strips for closing or opening the reclosable fastener elements. The slider includes depending side walls for maintaining the track structure and the rib and groove elements therebetween. The slider also includes separator structure inserted between the track structure but not the rib and groove elements to open the rib and groove elements. The zipper structure is slit or notched locally at the end where the slider goes when the zipper is closed, to allow the rib and groove elements to interlock fully making a leakproof closure.