Context: .The [Cii] 158 mum line is the most important coolant of the interstellar medium in galaxies but substantial variations are seen from object to object. The main source of the emission at a galactic scale is still poorly understood and candidates range from photodissociation regions (PDRs) to the cold neutral or diffuse warm ionized medium. Previous studies of the [Cii] emission in galaxies have a resolution of several kpc or more so the observed emission is an average of different ISM components.<BR /> Aims: .The aim of this work is to study, for the first time, the [Cii] emission at the scale of a spiral arm. We want to investigate the origin of this line and its use as a tracer of star formation.<BR /> Methods: . We present [Cii] and [Oi] observations of a segment of a spiral arm of M 31 using the Infrared Space Observatory. The [Cii] emission is compared with tracers of neutral gas (CO, Hi) and star formation (Halpha, Spitzer 24 mum).<BR /> Results: . The similarity of the [Cii] emission with the Halpha and 24 mum images is striking when smoothed to the same resolution, whereas the correlation with the neutral gas is much weaker. The [Cii] cooling rate per H atom increases dramatically from &tilde; 2.7 × 10<SUP>-26</SUP> erg s<SUP>-1</SUP> atom<SUP>-1</SUP> in the border of the map to &tilde; 1.4 × 10<SUP>-25</SUP> erg s<SUP>-1</SUP> atom<SUP>-1</SUP> in the regions of star formation. The [Cii]/FIR{42-122} ratio is almost constant at 2%, a factor 3 higher than typically quoted. However, we do not believe that M 31 is unusual. Rather, the whole-galaxy fluxes used for the comparisons include the central regions where the [Cii]/FIR ratio is known to be lower and the resolved observations neither isolate a spiral arm nor include data as far out in the galactic disk as the observations presented here. A fit to published PDR models yields a plausible average solution of G<SUB>0</SUB> &tilde; 100 and n &tilde; 3000 for the PDR emission in the regions of star formation in the arm of M 31.<BR />< Réduire