  It is well known that in the Sun, the frequencies and amplitudes of acoustic
modes vary throughout the solar cycle. Indeed, while the magnetic activity goes
towards its maximum, the frequencies of the modes increase and their amplitudes
decrease. We have analyzed data from the CoRoT mission on a few stars that
exhibit solar-like oscillations. The study of HD49933 (observed during 60 days
and 137 days spanning a total of 400 days) showed a modulation of the maximum
amplitude per radial mode and the frequency shifts of the modes, showing
magnetic activity in this rapidly rotating star. Moreover, both properties vary
in an anticorrelated way and the data allowed us to establish a lower limit for
the activity-cycle period of ~120 days. Measurements in Ca H and K lines
confirmed that this star is in the "active stars" category. We will also
discuss the results obtained for other targets such as HD181420 and HD49835 for
which we have investigated a similar behavior.
