In decapods, no analysis has ever been conducted of the force exerted by chelae, and the role that strength plays in the establishment of hierarchies. For this purpose, a device was designed to measure the force produced by the major chela in the freshwater crab Potamon fluviatile, where dominance hierarchy and agonistic behavioral patterns have previously been analyzed. Experiments showed that only in the females is strength the main factor influencing the outcome of contests. In males chela size more than chela strength is used to settle conflicts, together with other nonphysiognomic characteristics, molding the aggressive state of one individual, such as its history, including isolation from conspecifics, and possibly hormone concentration and metabolic levels.