Hitherto, the packing arrangement of the aquaporin-1 (AQP1) tetramer in a-dimensional (2-D) crystals (two-sided plane group p42(1)2) was observed to be largely similar (canonical crystal form) despite the difference in the source of the protein, the glycosylation state of the protein, the type of lipids, and the ratio of lipid to protein in the crystallization mixture. We report here our observation that the packing of AQP1 tetramers shows polymorphism in 2-D crystals generated in dioleoyl phosphatidylcholine bilayers. Apart from the canonical form, three additional allomorphs were identified. One was observed when small. (0.25) lipid to protein ratio was used in the crystallization mixture while the other two were observed when the divalent cation content in the canonical crystals was modified. The various allomorphs were distinguished by different relative orientations of the AQP1 tetramer viewed in projection. The same, two-sided plane group p4212 and similar unit cell dimensions were maintained in the different allomorphs as established by analysis of images of frozen-hydrated, nominally untilted crystals. Our results indicate that the interaction between the AQP1 monomers at the interface of the tetramers is flexible and is also strongly influenced by Mg2+ ions with the cation effect materializing because of the intrinsic fluidity of the membrane.