Crystalline molecular sieves with large pores and high adsorption capacities have many potential applications1, 2, 3, 4. Of these materials, zeolites are of particular interest owing to their stability in a wide range of experimental conditions. An aluminophosphate with very large circular channels5 containing 18 oxygen atoms (18-ring channels) has been synthesized, but in the search for large-pore zeolites, most of the materials which have been synthesized up to now contain only 14-ring channels; the synthesis of zeolites with larger ring structures has been believed to be hindered by the low Si-O-Si bond angles available. A silicogaloaluminate (ECR-34) with unidirectional 18-ring channels12 was recently reported, but exhibited low micropore volume, thus rendering the material less attractive for catalytic applications. Here we report the structure and catalytic activity of the silicogermanate zeolite ITQ-33; this material exhibits straight large pore channels with circular openings of 18-rings along the c axis interconnected by a bidirectional system of 10-ring channels, yielding a structure with very large micropore volume. The conditions for synthesis are easily accessible, but are not typical, and were identified using high-throughput techniques.