This study describes some aspects of the distribution, ecology, and rock boring behaviour of the shrimp Alpheus saxidomus Holthuis from the Pacific coast of Costa Rica. This species has a typical hammer-shaped dactylus on the first right pereiopod of females and males that shows tracks of abrasion at the hammer-head. Structural analysis of this hammer-dactylus proved that it is suited for the mechanical treatment of rocks. The very thick and calcified cuticle of the hammer is mostly formed by the sclerotinized, hard exocuticle, which has abundant pore canals.