The chicken c-ets-1 locus gives rise to two distinct transcription factors differing only in their structurally and functionally unrelated N-termini. One of these transcription factors, p54c-ets-1, contains a specific, short (27 amino acids), hydrophilic N-terminus encoded by a single exon, I54, that is widely conserved among vertebrates. The other one, p68c-ets-1, the cellular counterpart of the viral ets oncogene product, differs in the replacement of the I54 by two exons, termed alpha and beta, encoding a larger (71 amino acids), hydrophobic N-terminus which, in contrast to I54, exhibits properties of a transactivating domain. To date the alpha and beta exons have only been found in chicken. Here, we demonstrate the existence of the alpha and beta exons in other avian species (quail and duck) and the existence of the alpha exon in reptiles (turtle). However, none of them could be detected in mammals. Our results strongly suggest that, in contrast to the phylogenetically well-conserved I54 exon, the alpha exon is restricted to reptilian species (birds and 'true' reptiles), whereas the beta exon is detectable so far only in birds. Comparison of their amino acid sequences reveals that the alpha exon and to a much greater extent the beta exon have diverged faster than the I54 exon. In addition, we show that the N- and C-terminal thirds of the alpha exon and the highly hydrophobic nature of the alpha beta-encoded sequence are heavily conserved features and thus likely to be required for function as a transactivating domain in p68c-ets-1 and possibly in the viral P135gag-myb-ets transforming protein.