A well-known derivation of the group velocity for free-electron wave packets is generalized in order to include the effect of a periodic lattice potential and a uniform d.c. electric field. The result is an expression for the instantaneous position and shape of an electron wave packet as it accelerates in a one-dimensional crystal lattice with an applied field. A Mathieu-type lattice potential is used as an example to illustrate the outstanding features of the motion.