  We found evidence for the super-orbital modulation in the X-ray emission of
LS I +61 303 from the longest monitoring date by the RXTE. The time evolution
of the modulated fraction in the orbital light curves can be well fitted with a
sinusoidal function having a super-orbital period of 1667 days. However, we
have found a 281.8+/-44.6 day shift between the super-orbital variability found
at radio frequencies and our X-ray data. We also find a super-orbital
modulation in the maximum count rate of the orbital light curves, compatible
with the former results, including the shift.
