Earthworm casts deposited in burrows may stimulate plant roots to elongate preferentially tot hose burrows. A model experiment was conducted with an artificial, impenetrable subsoil in which vertical holes were left empty, filled with earthworm casts, or filled with fertile soil. The roots of perennial ryegrass had elongated down through an aggregate seedbed and were deflected horizontally when they reached the artificial subsoil. After six days, approximately 6% of the roots that were deflected sideways, met with holes in the subsoil. However, the number of roots that met with holes filled with either casts, or soil, was not significantly different from the number of roots that met with empty holes. Under the conditions of our model system, there was no evidence that ryegrass roots elongated preferentially to and entered holes filled with earthworm casts.