Crickets have traditional sex roles, where males compete aggressively for access to selective polyandrous females. However, in a laboratory experiment, we found that normally nonaggressive female Gryllus campestris fought each other vigorously in the presence of a courting male, resulting in a dominant female that gained a greater probability of receiving the spermatophore. Female-female fights included the same series of characteristic actions known from male-male fights, which demonstrates that the females can perform the full repertoire of agonistic actions except for the production of the aggressive rival song. Since females remained nonaggressive towards each other in the vicinity of a muted male, but were induced to fight each other in the complete absence of a male by the auditory experience of the courtship song, this song is both sufficient and necessary to induce female competition for males. Calling song was as effective as courtship song at inducing female aggression, whereas rival song was least effective. We therefore speculate that the calling and courtship songs may signal a male's resource value. (C) 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.