We present pulse phase-resolved X-ray spectra of the high mass X-ray binary Vela X-1 using the Rossi X-ray Timing Explorer .
We observed Vela X-1 in 1998 and 2000 with a total observation time of \sim 90 ksec .
We find an absorption feature at 23.3 ^ { +1.3 } _ { -0.6 } keV in the main pulse , that we interpret as the fundamental cyclotron resonant scattering feature ( CRSF ) .
The feature is deepest in the rise of the main pulse where it has a width of 7.6 ^ { +4.4 } _ { -2.2 } keV and an optical depth of 0.33 ^ { +0.06 } _ { -0.13 } .
This CRSF is also clearly detected in the secondary pulse , but it is far less significant or undetected during the pulse minima .
We conclude that the well known CRSF at 50.9 ^ { +0.6 } _ { -0.7 } keV , which is clearly visible even in phase-averaged spectra , is the first harmonic and not the fundamental .
Thus we infer a magnetic field strength of B = 2.6 \times 10 ^ { 12 } G .