An X-ray crystal structure of CYP2B4 in complex with the drug paroxetine [(3S,4R)-3-[(2H-1,3-benzodioxol-5-yloxy)methyl]-4-(4-fluorophenyl)piperidine] was solved at 2.14 Å resolution. The structure revealed a conformation intermediate to that of the recently solved complex with amlodipine and that of the more compact complex with 4-(4-chlorophenyl)imidazole in terms of the placement of the F-G cassette. Moreover, comparison of the new structure with 15 previously solved structures of CYP2B4 revealed some new insights into the determinants of active-site size and shape. The 2B4-paroxetine structure is nearly superimposable on a previously solved closed structure in a ligand-free state. Despite the overall conformational similarity among multiple closed structures, the active-site cavity volume of the paroxetine complex is enlarged. Further analysis of the accessible space and binding pocket near the heme reveals a new subchamber that resulted from the movement of secondary structural elements and rearrangements of active-site side chains. Overall, the results from the comparison of all 16 structures of CYP2B4 demonstrate a cluster of protein conformations that were observed in the presence or absence of various ligands.