In this paper, we examine two questions: (1) can the effects of transverse mixing of bacteria in a system constructed to have a permeability discontinuity in the direction parallel to the flow be measured; and (2) if the effects are measurable, can they be calculated using a transverse dispersion coefficient estimated from experiments using a conservative tracer? Pulses of chloride and bacteria were transported downward through heterogeneous columns constructed with a tubule of coarse, quartz sand surrounding an annulus of fine, quartz sand. Pulses of each were also transported through homogeneous columns of the two sands. Doubly peaked breakthrough curves resulted from the columns containing two distinct sand sizes. Modeling of the breakthrough curves was performed taking into account advection, dispersion, deposition, entrainment, and pore-size exclusion. The results revealed that transverse mixing does occur during transport of bacteria through heterogeneous material and that this mixing can be estimated using a conservative tracer. ¿ 1998 American Geophysical Union