Crystalline forms of 4-acetoxy-N,N-dimethyltryptamine (psilacetin), compositions containing that crystalline form, and their methods of use are disclosed. The crystalline forms of 4-acetoxy-N,N-dimethyltryptamine (psilacetin) according to the disclosure include crystalline 4-acetoxy-N,N-dimethyltryptammonium hydrofumarate, the fumarate salt of psilacetin, and/or crystalline bis(4-acetoxy-N,N-dimethyltryptammonium) fumarate, and pharmaceutical compositions containing a crystalline form of psilacetin.