The discovery of the striking catalytic properties of the Mobil zeolite ZSM-5 has been followed by a large and intensive research effort both in catalysis promoted by ZSM-5 and in the synthesis and properties of other highly siliceous zeolites. While the distinctions between ZSM-5 and some of the claimed new zeolites are barely perceptible (it would perhaps be invidious to name these zeolites), other new. highly-siliceous zeolites differ markedly from ZSM-5/ZSM-11 in structure and consequently in other properties as well. Zeolites Nu-1 and FU-1 have now been fairly well characterized in both physical and catalytic properties. Although their crystal structures have not yet been determined, it is clear that they are different from ZSM-5 (and indeed, any other published zeolite structure).