A tailored resolving agent for p-substituted 1-arylethylamines, which was designed on the basis of a criterion derived from our crystal-structure study, namely that the racemate and the resolving agent should have similar molecular lengths, is described. The designed enantiopure (2-naphthyl) glycolic acid (2-NGA) showed excellent resolving ability for a wide variety of p-substituted 1-arylethylamines.