In this paper, it will be shown that the isomorphism classes of regular orientable embeddings of the complete bipartite graph K n,n are in one-to-one correspondence with the permutations on n elements satisfying a given criterion, and the isomorphism classes of them are completely classified when n is a product of any two (not necessarily distinct) prime numbers. For other n, a lower bound of the number of those isomorphism classes of K n,n is obtained. As a result, many new regular orientable embeddings of the complete bipartite graph are constructed giving an answer of Nedela-Š koviera's question raised in [12]. ß 2005 Wiley Periodicals, Inc. J Graph Theory 50: [105][106][107][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122] 2005