I examined biased patterns of habituation that occur when reproductive male three-spined stickleback, Gasterosteus aculeatus, respond to a choice of two dummy females, one displaying the head-up posture of a sexually receptive female and an otherwise identical dummy lacking this sign stimulus (i.e. a dummy in a 'neutral' horizontal posture). Males initially courted both dummies about equally, but within 5 min, their courtship to the horizontal dummy began to habituate and was eventually replaced by attack behaviour, which increased about four-fold during the 1-h trial. Courtship to the head-up dummy continued unabated throughout the trial and by 5 min exceeded courtship to the horizontal dummy. Attack to the head-up dummy increased slightly in the first half of the trial then leveled off to about one-third that directed to the horizontal dummy. Both dummies were stationary and unreactive so could not directly provide differential feedback to the males. It is speculated that the self-arousal males obtain when responding to head-up posture reinforces their response to that cue. The salience of this sign stimulus may develop through an interactive process wherein the male's predisposition to recognize and court the head-up posture and the sexual self-arousal he obtains by doing so counter habituation and help maintain his sexual response to that cue. The stimulus-response specificity of male courtship is assumed to have an innate basis but requires experience to become fully differentiated. The results also illustrate that rate or extent of habituation to different stimuli, even on the same dimension, can differ within the same individual and in an adaptive manner. Copyright 2000 The Association for the Study of Animal Behaviour.