By low stringency screening of a Xenopus laevis oocyte cDNA library with a probe prepared from the yes oncogene of the avian retrovirus Y73, we have isolated clones encoding a 537 amino acid protein with 96% amino acid identity to the protein product of the human fyn proto-oncogene. In addition to the high degree of conservation at the amino acid sequence level, we also find a surprisingly high degree of sequence conservation between portions of the 5' untranslated regions of the frog and human genes. Probing of total DNA from a homozygous diploid frog with a fragment from one of the cDNA clones gave a hybridization pattern which is consistent with the presence of two fyn genes in the haploid Xenopus laevis genome. Hybridization analysis revealed that the ovary contains two different sized fyn RNAs, one 3.8 kb in length and the other 3.1 kb in length. Our finding of fyn transcripts brings to three the number of protein-tyrosine kinases of the src family whose mRNAs are found in X. laevis oocytes. This suggests that multiple protein-tyrosine kinases of the src family are required for processes in oogenesis or early development.