We present a study of theβ Cephei instability strip based on a sampleof 49 stars of this type. After deriving their effective temperatures and luminositiesfrom their observed (B - V), (U - B) colors and parallaxes we find their positionsin the HR diagram to be mostly confined to the main sequence, and their massesto lie between 7 M and 30 M . Their distribution on the HR diagram matcheswell with our previous theoretical instability strip which has an upper bound in theluminosity and rather tight boundaries in the effective temperature.