The name Macaronichnus segregatis is here proposed for a distinctive trace fossil that occurs in Jurassic and younger sand or sandstone. The trace fossil consists of a cylindrical smooth-sided, sinuous intrastratal trail, 0.3-0.5 cm in diameter, filled with sand slightly fighter in color than the host sand. Mica flakes or heavy minerals generally are concentrated around the margin. The trails interpenetrate but do not branch, tend to be more or less horizontal, and generally occur in dense concentration. The trace fossil has been found only in deposits thought, on the basis of other evidence, to be of shallow marine origin. A similar intrastratal trail occurs in modern intertidal and shallow subtidal sand in Willapa Bay, Washington, where it is produced by the marine polychaete, Ophelia limacina . This polychaete is a deposit feeder that largely sustains itself on bacteria and organic matter on the surface of sand grains. The trail is a feeding trace, apparently formed by the selective ingestion of grains with rough surface texture, which have a relatively larger surface area and potentially higher bacteria count than do smooth grains such as mica flakes, clay mineral aggregates, or certain heavy minerals. When engaged solely in locomotion, O. limacina produces a trace that disrupts layering but shows no mineralogic segregation. The genus, Ophelia , occurs in intertidal and shallow subtidal sands over a broad geographic range. The ancient examples of Macaronichnus segregatis , although not necessarily produced by Ophelia , were probably formed by a deposit feeding polychaete of similar size and shape. The depositional setting of the known occurrences of this trace fossil, and the production of the trace by a modern shallow marine polychaete, suggest that it may be an excellent indicator of ancient intertidal and shallow subtidal deposits.