Abstract,-Songs of Darwin's finches were studied on the Galapagos Island of Daphne Major from 1976 to 1995. A single, structurally simple, and unvarying song is sung throughout life by each male of the two common species, Geospiza fortis (medium ground finch) and G. scandens (cactus finch). Songs of the two species differ strongly in quantitative features, and individual variation among males is much broader in G.fortis than in G. scandens. Although there are exceptions, songs of sons strongly resemble the songs of their fathers. They also resemble the songs of their paternal grandfathers, but not their maternal grandfathers, indicating that they are culturally inherited and not genetically inherited. Female G. fortis display a tendency to avoid mating with males that sing the same type of song as their father. They also avoid mating with males that sing heterospecific song, with very rare exceptions. Thus song, an evolving, culturally inherited trait, is an important factor in species recognition and mate choice. It constrains the mating of females to conspecifics, even when there is no genetic penalty to interbreeding, and thus may playa crucial role in species formation by promoting genetic isolation on secondary contact. The barrier is leaky in that occasional errors in song transmission result in misimprinting, which leads to a low incidence of hybridization and introgression. Introgression slows the rate of postzygotic isolation, but can produce individuals in novel genetic and morphological space that can provide the starting point of a new evolutionary trajectory.Key words.-Hybridization, imprinting, inbreeding avoidance, mate choice, speciation, species recognition.Received November 28, 1995. Accepted April 30, 1996.Bush (1993) has emphasized that complete reproductive isolation is the end point of the speciation process, not the beginning, thereby drawing attention to the period when diverging taxa are still capable of interbreeding. In birds, this potential to hybridize may remain for 20 million yr or more after divergence from a common ancestor (Prager and Wilson 1975). Indeed, a low incidence of hybridization is known to occur in approximately one in 10 species of birds (Grant and Grant 1992a). To understand the process of speciation and the consequences of introgression we need to know the nature of the barrier to gene flow in sympatry and the extent to which it is permeable. Species of Darwin's finches on the Galapagos are suitable for this purpose. Having diverged from a common ancestor approximately 3 M.Y.B.P. (Grant 1994), they are closely related, morphologically distinct, yet capable of interbreeding. Interbreeding species are similar in courtship behavior and plumage, but differ markedly in body and bill size and shape and in song. In this paper we examine the role of song as a barrier to gene exchange in sympatry.Studies of bird song over the last 40 years have provided observational and experimental evidence that species can discriminate between conspecific and heterospecific song (Marler