  This paper is concerned with the propagating speeds of transition fronts in
$R^N$ for spatially periodic bistable reaction-diffusion equations. The notion
of transition fronts generalizes the standard notions of traveling fronts.
Under the a priori assumption that there exist pulsating fronts for every
direction $e$ with nonzero speeds, we show some continuity and
differentiability properties of the front speeds and profiles with respect to
the direction $e$. Finally, we prove that the propagating speed of any
transition front is larger than the infimum of speeds of pulsating fronts and
less than the supremum of speeds of pulsating fronts.
