We present the results of a Beppo SAX observation of the fastest pulsar known : PSR B1937+21 .
The \sim 200 ks observation ( 78.5 ( 34 ) ks MECS ( LECS ) exposure times ) allowed us to investigate with high statistical significance both the spectral properties and the pulse profile shape .
The absorbed power law spectral model gave a photon index of \sim 1.7 and N _ { H } \sim 2.3 \times 10 ^ { 22 } cm ^ { -2 } .
These values explain both a . the ROSAT non-detection and b . the deviant estimate of a photon index of \sim 0.8 
obtained by ASCA .
The pulse profile appears , for the first time , clearly double peaked with the main component much stronger than the other .
The statistical significance is 10 \sigma ( main peak ) and 5 \sigma 
( secondary peak ) .
The 1.6–10 keV pulsed fraction is consistent with 100 % ; only in the 1.6–4 keV band there is a \sim 2 \sigma indication for a DC component .
The secondary peak is detected significantly only for energies above 3 \div 4 keV .
The unabsorbed ( 2–10 keV ) flux is F _ { 2 - 10 } = 3.7 \times 10 ^ { -13 } erg cm ^ { -2 } s ^ { -1 } , implying a luminosity of L _ { X } = 4.6 \times 10 ^ { 31 } \Theta ( d / 3.6 kpc ) ^ { 2 } 
erg s ^ { -1 } and an X-ray efficiency of \eta = 4 \times 10 ^ { -5 } \Theta , where \Theta is the solid angle spanned by the emission beam .
These results are in agreement with those obtained by ASCA .