  We measure rotation periods for 12151 stars in the Kepler field, based on the
photometric variability caused by stellar activity. Our analysis returns stable
rotation periods over at least six out of eight quarters of Kepler data. This
large sample of stars enables us to study the rotation periods as a function of
spectral type. We find good agreement with previous studies and vsini
measurements for F, G and K stars. Combining rotation periods, B-V color, and
gyrochronology relations, we find that the cool stars in our sample are
predominantly younger than ~1Gyr.
