Differences in the song repertoires of males and in the song-pools of the thrush nightingale populations were studied in 1972, 1983 and 1984 in southern Finland. Changes in male repertoires and in the song-pool of one population were monitored in 1972 and 1980-1985. The thrush nightingales in a local population had repertoires that were more similar to each other than to those of the males in other local populations. The similarity of the repertoires decreased with increasing distance, but there were no clear-cut dialect boundaries between local populations. The song repertoire of a male was more similar to that of the adjacent males than to his own repertoire of the previous year. The similarity of the repertoires of adjacent singers increased during the singing period. This similarity was partly due to the same song-types being used with about equal frequency, but obviously the males were also able to learn new songs from their adjacent singers. After dispersal to a breeding area thrush nightingales learn at least some new song-types, even at the age of two to four years. Some old breeders were able to copy new song-types from the immigrants (mostly young males) or the playback tape. The newly copied song-types were loud and simple in structure, whereas the song-types soon to be abandoned were weak in amplitude. In one population, studied from 1980 to 1985, the repertoires of the males tended to become more similar in successive years. This tendency, however, did not occur in 1984 when the proportion of immigrants in the population was unusually high. The major changes in local song traditions were due to "cultural diffusion" by males originating from areas with other traditions; this diffusion greatly enriched the local song-pool, especially in years when the rate of immigration was high.