A method for computing isovalue or contour surfaces of a trivariate function is discussed. The input data are values of the trivariate function. F[?] [?] k, at the cuberille grid points (x[?], y[?] , zk) and the output is a collection of triangles representing the surface consisting of all points where F(x, y, z) is a constant value. The method described here is a modification that is intended to correct a problem with a previous method.