A crystalline zeolite designated ZSM -12 having the following composition expressed as mole ratios of oxides:1.0 .+-. 0.4 M.sub.2/n O .sup.. W.sub.2 O .sup.. 20-100 YO.sub.2 .sup.. ZH.sub.2 Owhere M is at least one cation and n is the valence thereof, W is aluminum or gallium, Y is silicon or germanium, and Z is a value ranging from 0 to 60. In a preferred synthesized form, M is a mixture of alkali metal cations, especially sodium, and tetraalkylammonium cations. These zeolites are characterized by a specified X-ray powder diffraction pattern. Catalytically active forms of said zeolites are used in a wide variety of hydrocarbon conversion reactions.