The "ovalbumin Y" gene, one of three which constitute the ovalbumin gene family in chicken has been completely sequenced. The exact location of exons can be derived from the comparison with the ovalbumin gene sequence and from the map previously established by electron microscopy analysis. During evolution of the Y gene, selective pressure has operated to retain a sequence coding for an ovalbumin-like protein. The location of splice junctions, the length of protein coding exons and the reading phase are as in the ovalbumin gene. The overall homology between the Y and ovalbumin protein coding sequences is 72.6% (resulting in a 58% homology for the amino acid sequences). A significantly high number of base changes within coding sequences are present in clusters, which appear in several cases to be correlated with the occurrence of direct repeats. The 3' untranslated sequences of the Y and ovalbumin mRNAs have diverged much more, and the Y sequence contains a peculiar U(T) rich region. Corresponding introns of the ovalbumin and Y genes differ extensively both in sequence and in length. They share however characteristic biases in their base distribution.