（Drawing by Program MOLSCRIPT）The inclusion complex of a-cyclodextrin (a-CD)/4,4'-biphenyldicarboxylic acid (BPDC) was crystallized into two crystal forms; tetragonal, space group P42212, a = b = 19.609(8)A, c = 32.257(8)A (Form 1), and orthorhombic, space group P21212, a = 35.397(8)A, b = 24.577(5)A, c = 27.969(9)A (Form 2). Both crystal structures were determined by X-ray diffraction method. The results show that a guest BPDC was included in two a-CDs in the tail-to-tail orientation to form the 2:1 a-CD/BPDC inclusion complex, and that a BPDC interacts with another BPDC by guest-water-guest hydrogen bonds. In Form 1, the a-CDs are arranged in a continuous one dimensional column structure, whereas in Form 2 they are arranged in a discontinuous. (Kamitori et al. (1998) Carbohydrate Research 312, 177-181)