Chemosensory proteins (CSPs) are expressed in many insect sensory appendages and are thought to be involved in chemical signaling by ants. Using a pattern search of the Lausanne fire ant EST library, we identified at least eleven unique CSP sequences in Solenopsis invicta. We compared these sequences with other known hymenopteran CSPs in a phylogenetic tree and found one sequence similar to Apis mellifera AmelCSP4, which is known to be expressed exclusively in the antenna. Thus, it is a good candidate for an odorant- or pheromone-binding protein. A cluster of five fire ant CSP sequences has no close hymenopteran homologs. We found that one member of this group (SinvCSP1) is highly expressed in worker antennae, suggesting an olfactory function for these CSPs. We cloned the coding sequence for SinvCSP1, expressed it from E. coli, and purified it by ion exchange and hydrophobic interaction chromatography. Purified SinvCSP1 binds to 12-bromododecanoic acid with a dissociation constant of 1 μM, and to N-phenyl naphthylamine (NPN) with a dissociation constant of 10 μM. A pentane extract of fire ant cuticle was found to strongly inhibit NPN binding, suggesting cuticular substances are ligands for this CSP.