Abstract: As synthesized by conventional technique, zeolite ZSM-5 crystals are cubic with some twinning and measure in size over the range of from about 3 .times. 3 microns to about 8 .times. 8 microns. By synthesizing crystalline aluminosilicate zeolite ZSM-5 according to the present method, i.e. in the presence of a tetraureacobalt (II) complex and with a specifically defined reaction mixture composition, the crystalline product will consist of highly twinned rectangular prismatic crystals exhibiting extreme uniformity in size of from about 5 .times. 10 microns to about 10 .times. 20 microns, depending on the ratio of said tetraureacobalt (II) complex to silica in the zeolite synthesis reaction mixture. Synthesis of ZSM-5 in accordance hereto will provide a zeolite with enhanced catalytic activity for certain chemical reactions.