Prior studies have shown that colloids can facilitate contaminant migration in unimodal porous media. To investigate the effect of no-flow regions on flow and contaminant transport in dual-porosity soils, we model a porous medium composed of two different homogeneous, superposed, and interacting regions: the mobile region and the immobile region. We assume that the advective-dispersive processes govern the transport of contaminant and colloids in the mobile region, while the diffusion process dominates in the immobile region. The contaminant and colloid mass transfer mechanisms between these two regions are represented by a first-order mass transfer. Colloid deposition on the solid matrix is expressed by a kinetic sorption relationship. The contaminant sorption with the solid matrix and colloidal surfaces is also incorporated into the model. Coupled with mass transfer terms, two sets of governing equations representing the fate and transport of contaminant and colloids in both the mobile and immobile regions are developed and applied to experimental data available in the literature. Numerical solutions are obtained by employing a fully implicit, finite difference scheme. The numerical results indicate that the colloidal facilitation is increased in a dual-porosity porous medium compared to a unimodal medium. The model is validated by comparing the numerical results with the experimental data available in the literature for colloid-facilitated contaminant transport in a single-porosity medium and contaminant transport in a colloid-free, dual-porosity medium. A sensitivity analysis is conducted to deduce the effect of major model parameters on contaminant transport. The analysis demonstrates that, although both the volumetric fraction of the mobile region and the mass transfer rate coefficients between the two regions have effects on the dual-porosity transport, the early breakthrough is affected mainly by the volumetric fraction of the mobile region, while the tailing is affected largely by the mass transfer rate coefficients. ¿ 1999 American Geophysical Union