Organic and inorganic compounds infiltrating into groundwater are subject to redox reactions catalyzed by bacteria. These bacteria tend to form conglomerates and to adsorb to the solid matrix. Assuming a biofilm concept, the mass transport in and near such a biofilm is a controlling factor for different processes affecting the microbial activity, such as the supply of the substrates to the bacteria or the detachment of biofilm parts by shear forces. We present here a macroscopic model for the transport of dissolved substances in groundwater-biofilm systems. This model conceptualizes the diffusion dominated microscopic transport processes within the biofilm by using a logistic approach based on a diffusional limitation. As biological processes, the growth, the maintenance, and the decay of the bacterial population are explicitly accounted for. The model is formulated for a one-dimensional flow and transport field and is numerically solved with an operator splitting technique as described by Zysset et al. (1994). It is able to reproduce the transport of nitrate and sulfate in two laboratory column experiments in which these substances served as dissolved electron acceptors in bacterially mediated redox reactions. ¿ American Geophysical Union 1994