Analyzing IUE ultraviolet spectra of β Cep pulsating stars we noticed that multiperiodic variables have a larger mean metal abundance in the photosphere, [ m/H] , than monoperiodic ones. We apply statistical tests to verify this dichotomy. We obtain that, with a large probability, the multiperiodic β Cep stars have greater values of [ m/H] . This result is consistent with the linear non-adiabatic theory of pulsation of early B-type stars.