The K protein is among the major pre-mRNA-bindlng proteins (hnRNPs) in vertebrate cell nuclei. It binds tenaciously to cytidine-rich sequences and is the major oligo(rC/dC)-binding protein in vertebrate cells. We have cloned a cDNA of the Xenopus laevls hnRNP K and determined its sequence. The X.laevls hnRNP K Is a 47 kD protein that is remarkably similar to its human 66 kD counterpart except for two large internal deletions. The sequence of hnRNP K contains a 45 amino acid repeated motif which is almost completely conserved between the X.laevls and human proteins. We found that this repeated motif, the KH motif (for K homology), shows significant homology to several proteins some of which are known nucleic acids binding proteins. The homology is particularly strong with the archeabacterlal ribosomal protein S3 and with the saccharomyces cerevlslae protein MER1 which is required for melosls-speciflc splicing of the MER 2 transcript. As several of the proteins that contain the KH motif are known to bind RNA, this domain may be involved in RNA binding.