Horseshoe crabs come ashore in attached pairs during spring high tides to mate and nest on beaches of the Atlantic and Gulf coasts. Unattached males also come ashore and crowd around the nesting pairs as satellites and engage in sperm competition with the attached male. Females with no satellites and females with large numbers of satellites nest next to one another on the same tide. When females are removed and replaced by a cement model, satellite males continue to be attracted to the same location. Models over sites where females with many satellites had nested are more attractive to males than sites from which a female with no satellites had been removed or a site where no crab had been nesting recently. A second experiment demonstrated that males are responding to chemical cues. A sponge filled with seawater taken from below a female with many satellites and placed under a model female was more attractive to males than a sponge filled with seawater. This is the first demonstration that horseshoe crabs use chemical cues, in addition to visual cues, to locate mates.