Abstract. In this paper we study semilocal knots over/into £, that is, embeddings of a manifold N into £(£). the total space of a 2-disk bundle over a manifold M, such that the restriction of the bundle projection^: E(£) -M to the submanifold N is homotopic to a normal map of degree one, f: N -N.We develop a new homology surgery theory which does not require homology equivalences on boundaries and, in terms of these obstruction groups, we obtain a classification (up to cobordism) of semilocal knots over/ into £.In the simply connected case, the following geometric consequence follows from our classification. Every semilocal knot of a simply connected manifold M#K in a bundle over M is cobordant to the connected sum of the zero section of this bundle with a semilocal knot of the highly connected manifold K into the trivial bundle over a sphere.Introduction. It is a well-known fact that complex hypersurfaces with isolated critical points give rise to embeddings of highly connected manifolds in spheres, that is, (real) codimension two submanifolds K of S2k+l with trt(K) -0 for i < k -1 [M]. To study such embeddings, it is natural to consider the techniques employed in the classification of classical knot theory and the higher-dimensional analoguesembeddings of S" into Sn+2. One approach is that of surgery theory. As one application of T-homology surgery theory, Cappell and Shaneson obtained a calculation of the knot cobordism groups [CS]. The more general problem classifying highly connected codimension two embeddings up to cobordism cannot be answered in terms of known surgery theories. One intent of this paper is the development and application of a new surgery theory, B-surgery, which is useful when studying geometric problems in which the dominant maps are not homotopy or homology equivalences. The maps derived from these situations will have the property that the restriction to the boundary does not induce a homology equivalence, not even over