The white-throated sparrow is rapidly becoming an important model in the genetics of social behavior because of a chromosomal rearrangement that segregates with a behavioral phenotype. Within a population, 50 % of individuals are heterozygous for a rearranged chromosome 2 (ZAL2(m)). These birds sing more and are more aggressive than the other 50 %, who lack the rearrangement. A disassortative mating system, in which heterozygotes almost never interbreed, ensures that ZAL2(m)/2(m) homozygotes are extremely rare. Here, we provide the first systematic characterization of such a homozygote, a hatch-year female. Her plumage was atypical of her age and sex, resembling that of an adult male. She was extremely vocal and aggressive, dominating her opponents in behavioral tests. Her phenotype was thus an exaggerated version of a typical ZAL2/2(m) heterozygote, supporting the hypothesis that alleles inside the ZAL2(m) rearrangement confer high aggression and further emphasizing this species' value as a model of social behavior.