Transport of bacteria in a soil was delayed relative to the mobile pore water by rate-limited rather than instantaneous sorption to soil particles. Desorption rates were found to be faster than adsorption rates, and sorption kinetics were dependent on the density of bacteria and on the flow rate of water. Sorption kinetics were revealed by a combination of model descriptions of the mass balance and sorption kinetics of a groundwater bacterium entering a porous system and laboratory experiments using a stirred flow chamber and a soil column. The flow chamber was designed and operated to (1) distinguish instantaneous equilibrium reactions from time-dependent reactions, (2) estimate sorption rate coefficients at different cell densities and water velocities, and (3) obtain rate coefficients that could be used to predict the breakthrough of bacteria in a soil column under saturated, continuous flow conditions. Sorption rates in the stirred flow chamber were expressed by three rate laws, first order, Langmuir, and second order, and the transport of bacteria through the soil column was described and predicted by a model assuming a mixture of instantaneous and time-dependent sorption reactions and, alternatively, by a kinetic model. The sorption rate was primarily dependent on the density of sorbed bacteria, but the desorption rate was, in addition, influenced by the density of cells in the aqueous phase. Rate coefficients obtained in the flow chamber could be used to accurately predict transport of bacteria in the soil column at the corresponding density and flow rate, but extrapolations are complicated by the nonlinear relationship between density and sorption rate. Timescales for reaching equilibrium estimated from the rate coefficients were in the range from 10--15 hours to several days, longer for low densities and velocities. ¿ American Geophysical Union 1995