This specification discloses, as a composition of matter, a phosphorus-containing zeolite and discloses a method for preparing it. The composition is a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12, a constraint index of about 1 to 12, and containing phosphorus incorporated with the crystal structure thereof in an amount of at least about 0.78 percent by weight. The specification also discloses the use of the composition as a catalyst for the conversion of aliphatic compounds.