Anartia fatima and A. amathea form a hybrid zone in Panama where F1 and back–cross hybrids are found. Crosses were carried out to determine the nature of any reproductive isolation between these two butterflies. A novel analysis demonstrated both strong assortative mating among the pure forms and an unusual example of Haldane's rule: F1 hybrid females (the heterogametic sex) from the cross A. amathea (female) multiplied by A. fatima (male) have a reduced tendency to mate. Historically, Haldane's rule has been restricted to hybrid mortality or sterility and most studies have concentrated on taxa (predominantly Drosophila) between which strong barriers to gene flow already exist. Our data suggest that Haldane's rule might be extended to cover any decrease in hybrid fitness and that mating propensity may provide a sensitive and comparable means of assessing such decreases. Other barriers to gene flow were also evident in Anartia: F1 hybrid females have reduced fertility (also a Haldane effect) and larval survivorship was greatly reduced in F2 hybrids of both sexes. These examples of hybrid disruption are expected under the dominance theory of Haldane's rule but do not exclude other explanations.