A three-dimensional analytical model is developed to quantify the process of oxygen-limited biodegradation as it occurs at field scales. The model incorporates the effects of chemical and microbiological heterogeneities inherent to the biodegradation process in a stochastic analysis of the coupled transport equations for a system consisting of a contaminant and an oxidizing agent, that is, oxygen, in heterogeneous and anisotropic aquifers. Natural aquifer variability, equilibrium linear sorption and Monod-type kinetics for the microbial population constitute the sources of these heterogeneities. Calculations for hypothesized field conditions show that longitudinal macrodispersivities for the contaminant and dissolved oxygen are considerably affected by biodegradation. The effective contaminant decay rate is fundamentally found to be moderately less than the mean, while the effects are lesser for the effective retardation factor. The usual assumption of equal dispersivities for both components is found to be inadequate over most concentration ranges.¿ 1997 American Geophysical Union