Using a variety of search strategies, we obtained the complete or nearly complete repertoire of trace amine receptors from humans, mice, rats, zebrafish, pufferfish, and a number of invertebrates. We found that the number of functional receptors varies from 5 to 50 in each genome, showing that this family of GPCRs has a very dynamic gene repertoire. We show that the previously cloned and characterized GPCRs from insects and mollusks are more closely related to mammalian serotonin, dopamine, and adrenalin receptors than to mammalian TA receptors. This suggests that the ability to bind TAs has arisen independently in different developmental lineages.