We observed 21 polarized background radio sources in the field of M 31
at 1.365 GHz and 1.652 GHz, acid determined their rotation measures (
RMs). The RM data show that the regular magnetic field of the disk pro
bably extends from about 5 kpc to 25 kpc from the center with similar
structure. The RMs obtained from the polarized emission from M 31 at l
ambda 6 cm and lambda 11 cm indicate that M 31 might have a weak poloi
dal field in its tenuous halo. Observational features of the odd and e
ven dynamo modes in a galaxy are discussed. An even mode (S0) dynamo m
ay operate in M 31.