The effectiveness of septic system filter fields can be reduced by preferential flow through soil macropores. Burrows of anecic earthworms may contribute to this concern by penetrating the infiltrative surface of soil-wastewater-treatment-system trenches. Adding a layer of sand on the bottom of the trench, however, may discourage earthworm burrowing. In this laboratory study we investigated if the particle size distribution and position of a sand layer would inhibit burrowing by the anecic earthworm Lumbricus terrestris L. Three earthworms were added to the top of soil columns containing a 3-cm-thick sand layer (various particle size distributions tested) covered with 2 mm of loamy B-horizon material and underlain by 9 cm of the same material. We repeated this experiment with earthworms placed below the sand layer. In another experiment, the sand layer was covered by 11.3 cm of soil material to investigate whether worms added to the surface would avoid crossing a sand layer embedded within the columns. No treatments in the surface sand layer experiments prevented all worms from establishing burrows. The worms added to the surface, however, took an average of 1.7 to 5 times longer (P °Ü 0.05) to establish burrows and earthworm mortality was greater in columns with a surface sand layer compared to the no-sand-layer controls. In the embedded-sand-layer experiment, no worms burrowed past the soil-sand interface, whereas all worms in the controls burrowed past the equivalent depth. Within the range of materials investigated, the particle size distribution of the sand had no significant affect on earthworm burrowing. The results suggest that adding a layer of sand to the bottom of soil-treatment-system trenches may deter earthworms, but will not prevent them from burrowing if their survival depends on it.