A cross between queen butterflies of the Palaeotropical species Danaus chrysippus and the Neotropical D. gilippus was achieved with difficulty in both directions. Only one progeny (N = 70) was reared comprising sterile males and inviable females in a precisely 1 : 1 ratio. Both prezygotic and postzygotic barriers to gene flow are strong. The result supports Haldane's Rule, to which we propose a minor amendment. The F1 hybrids were intermediate for background colour between the brown (genotype BB) of gilippus and orange (genotype bb) of chrysippus. Most F1 pattern characters were also intermediate. In polymorphic chrysippus populations, because Bb heterozygotes are brown, or nearly so, we suggest the B allele may have evolved towards dominance in sympatry. Hybrid males show positive heterosis for body size. The close similarity of male genitalia between the allopatric, genetically distant species chrysippus and gilippus, compared to their divergence between gilippus and its largely sympatric sister species eresimus, suggest that reinforcement of sexual isolation or reproductive character displacement have evolved in sympatry.