The corallivorous filefish Oxymonacanthus longirostris is a predominantly monogamous species without parental care. Polygyny reduces female fitness because monogamous females benefit from male assistance with feeding–territory defense that secures both a feeding area and time to feed. I studied the strategy of females to prevent polygyny, with the help of field observations and manipulative field experiments. Monogamous females always behaved aggressively towards intruding floater females. When a caged male or female was placed within the territory of a monogamous pair, the resident female was more likely to pay attention to the caged female than to the caged male. Experimentally transplanted floater females succeeded in settling as secondary females in the monogamous territory more frequently when the resident female was caged. These results suggest that monogamous females behave aggressively towards floater females to defend their mating status; such aggressive behavior is successful in preventing polygyny. Furthermore, the removal experiment showed that polygynous (especially secondary) females moved to territories of widowed males located near them more frequently than monogamous females did. This outcome indicates that polygynous females can change their mating status and escape polygyny by moving to another territory if the opportunity occurs. Thus, female behavior can play an important role in the maintenance of monogamy and in lowering the level of polygyny within populations of this species.