Chemotaxis of the bacterium Escherichia coli and the planarian Dugesia dorotocephala toward a variety of substances impregnated in sustained-release polymers has been demonstrated. Tests were carried out in soft agar and in aqueous medium with highly consistent results. The simplicity of this method may make it particularly useful for rapid qualitative screening of new wild-type and mutant strains of bacteria for chemotaxis toward nonmetabolites. The cost, ease of fabrication, and the facility with which they can be utilized in test systems suggest that these sustained-release vehicles will provide a new and general method of creating concentration gradients to study chemotaxis.