A catalyst based on crystalline aluminosilicates of the pentasil type, having an Si/Al atomic ratio of at least 10, has the structure of primary crystallites of a mean diameter of at least 0.1 micron and at most 0.9 micron, which crystallites are partially combined into agglomerates the primary crystallites and/or agglomerates being mutually joined by finely disperse alumina obtainable by hydrolysis of aluminum-organic compounds, the BET surface area of the catalyst being 300 to 600 m.sup.2 /g and its pore volume (determined by mercury porosimetry) being 0.3 to 0.8 cm.sup.3 /g.