In this paper we propose a novel visibility-culling technique for optimizing the computation and rendering of opaque isosurfaces. Given a continuous scalar field f(x) over a domain D and an iso-value w, our technique exploits the continuity of f to determine conservative visibility bounds implicitly, i.e., without the need for actually computing the isosurface f^{ - 1} (w). We generate Implicit Occluders based on the change in sign of f*(x) = f(x) - w, from positive to negative (or vice versa) in the neighborhood of the isosurface. Consider, for example, the sign of f* along a ray r cast from the current viewpoint. The first change in sign of f* within D must contain an intersection of r with the isosurface. Any additional intersection of the isosurface with r is not visible. Implicit Occluders constitute a general concept that can be exploited algorithmically in different ways depending on the framework adopted for visibility computations. In this paper, we propose a simple from-point approach that exploits well-known hardware occlusion queries.