Animal signals are often used in more than one context or by more than one class of signaller. In the big-clawed snapping shrimp, Alpheus heterochaelis, the open chela display is a visual signal produced by both males and females. Thus, to respond appropriately to the open chela display in interactions, shrimp need information about the sex of the other individual. These experiments were performed to determine the effect of sex information from chemical signals on the response of shrimp to the visual signal of the open chela. First, chemical signals from males and females were presented alone. Shrimp did not respond differentially to chemical signals from males and females. Next, chemical signals were presented concurrently with the open chela display. Male shrimp showed a greater response to an open chela with male chemical signals than to an open chela with female chemical signals. Male response to an open chela with female chemical signals was also a function of the chela's relative size, and this size-relative response was less than the response to the open chela alone. Male response to an open chela with male chemical signals was independent of relative size. In contrast, females responded aggressively to open and closed chelae regardless of relative size, and regardless of concurrent chemical signals. The relative costs and benefits associated with aggressive and pair-forming interactions differ for males and females, and these differences are likely to contribute to the differences in assessment observed here.