A series of bicyclic arylamino/heteroarylamino hydroxamic acids (7–31) have been examined as novel histonedeacetylase6 (HDAC6) inhibitors. One compound (13) exhibits remarkable inhibitory activity of HDAC6 with an IC50 value of 0.29 nM, which is 4,000–43,000 times more selective over other HDAC isoforms. Compound 13 was shown to have antiproliferative activity against human multiple myeloma RPMI