Porous aluminosilicates of the zeolite type have become important industrially as ion-exchangers, molecular sieves and catalysts1,2. Topologically, the tetrahedral centres of the oxygen tetrahedra lie at the nodes of various 4-connected three-dimensional nets. Four-connected porous frameworks (AlPO4) composed of alternating AlO4 and PO4 tetrahedra are the basis of a new family of synthetic molecular sieves, which may become important industrially because they lack the strong ion-exchange and catalytic properties of the zeolites3. Aluminophosphates display a wide range of crystal structures because Al can be coordinated to either four, five or six oxygen atoms, some of which may belong to either hydroxyl or water species3–12. As part of a systematic study of the crystal chemistry of aluminophosphates, we have found that the framework of synthetic-phase H3 (ref. 13) (AlPO4 · 1.5 H2O) contains PO4 tetrahedra alternating between AlO4 tetrahedra and AlO4(H2O)2 octahedra. As reported here, the 4-connected three-dimensional net formed by linking adjacent Al and P atoms is a new type containing 6.6.6 and 4.8.8. two-dimensional nets joined by up–down linkages. This coexistence of two types of two-dimensional· nets in a three-dimensional net suggests that the principle of parsimony should not be adopted too strictly in attempts to invent new nets of potential relevance to molecular sieve technology.