A kinetic transport model with the ability to account for variations in cell density of the aqueous and solid phases was developed for bacteria in porous media. Sorption kinetics in the advective-dispersive-sorptive equation was described by assuming that adsorption was proportional to the aqueous cell density and the number of available sites on the solid phase, whereas desorption was proportional to the density of sorbed cells. A numerical solution to the model was tested against laboratory column data, and the performance was compared with that of a two-site model. The kinetic model described the column data as well as the two-site model did, but the highest efficiencies of both models were associated with experiments with the smallest sorption. Furthermore, the kinetic model accounted for cell density dependent sorption, as demonstrated by fair predictions of bacterial transport at one cell density when using parameters obtained at another cell density. 1978; Shales and Kumarasingham, 1987; Reynolds et al., 1989; Gannon et al., 1991; Fontes et al., 1991; Tan et al., 1991; Trevors et al., 1991], and some have used mathemat-Paper number 94WR01725. 0043-1397/94/94WR-01725505.00 ical models to generalize the results [Taylor and Jaffd, 1990; Lindqvist and Bengtsson, 1991; Hornberger et al., 1992; Lindqvist and Enfield, 1992b]. The models describe bacterial movement by the advective-dispersive equation modified to account for retention processes, growth, and mortality (see Corapciouglu and Haridas [1984] and Harvey [1991] for a discussion of these and additional processes). The cell velocity may be retarded compared with the water velocity by several retention processes, namely, sorption, physical straining, sedimentation, and interception. These processes lead to a distribution of cells between the solid phase of the medium and the pore water. The growth and mortality rates may be different in the solid and aqueous habitats [DeIaquis et al., 1989], and the distribution of cells is affected by organic matter content on the sand and in the water [Scholl and Harvey, 1992], nutrient concentrations [Bengtsson, 1989; Lindqvist and Bengtsson, 1991], cell density [Lindqvist and Enfield, 1992b], physiological status [Fletcher, 1977], ionic strength [Gannon et al., 1991; Fontes et al., 1991], pH [Scholl and Harvey, 1992], grain size [Fontes et al., 1991; Lindqvist and Enfield, 1992b], and oxyhydroxide coatings