  From the longest monitoring of LS I +61 303 done to date by the Rossi X-ray
Timing Explorer (RXTE) we found evidence for the long-sought, years- long
modulation in the X-ray emission of the source. The time evolution of the
modulated fraction in the orbital lightcurves can be well fitted with a
sinusoidal function having a super-orbital period of 1667 days, the same as the
one reported in non-contemporaneous radio measurements. However, we have found
a 281.8 \pm 44.6 days shift between the super-orbital variability found at
radio frequencies extrapolated to the observation time of our campaign and what
we found in the super-orbital modulation of the modulated fraction of our X-ray
data. We also find a super-orbital modulation in the maximum count rate of the
orbital lightcurves, compatible with the former results, including the shift.
