For integers p, q, s with p ≥ q ≥ 2 and s ≥ 0, let K−s 2 (p, q) denote the set of 2−connected bipartite graphs which can be obtained from Kp,q by deleting a set of s edges. F.M.Dong et al. (Discrete Math. vol.224 (2000) 107–124) proved that for any graph G ∈ K−s 2 (p, q) with p ≥ q ≥ 3 and 0 ≤ s ≤ min{4, q−1}, then G is chromatically unique. In this paper, we shall extend this result to p ≥ q ≥...