  There is much interest in the physics of materials that show a strong
coupling between magnetic and electric degrees of freedom. In a recent paper by
Mostovoy a theory is presented that is based on symmetry arguments and leads to
quite general claims which we feel merit some further analysis. In particular,
Mostovoy concludes that "spiral magnets are, in general, ferroelectric". We
argue that this conclusion is not generally valid, and that the symmetry of the
unit cell has to be taken into account by any symmetry-based magneto-electric
coupling theory. In an attempt to avoid further confusion in the search of new
multiferroic materials, we identify in this Comment some of the necessary
symmetry properties of spiral magnets that can lead to ferroelectricity.
