Field studies and numerical simulations suggest that solute transport in fractured rock occurs primarily along preferred pathways or channels that involve only a small percentage of the available fracture network. By examining relationships between path length and fluid velocity in a series of discrete network simulations, we conclude that mass tends to travel longer distances in fractures in which the fluid velocity is higher, although the longest paths traveled do not correspond to the highest fluid velocities within the network. These observations suggest that a velocity optimization process, operating at fracture intersections, causes mass channeling at the network scale. In networks composed of fractures of varying aperture, this mechanism explains the increasingly tortuous paths traveled as mass moves selectively through the larger apertures. ¿ American Geophysical Union 1995