The genes encoding FcRgamma and TCRzeta homologs were identified using a bioinformatic approach in the amphibian Xenopus laevis. Deduced amino acid sequence of Xenopus TCRzeta is highly similar to the mammalian and avian counterparts, whereas that of FcRgamma differs by the presence of an additional ITAM-like motif. The presence of the negatively charged residue in the transmembrane regions of both subunits suggests their ability to serve as signal transducing modules in complex with activating receptors. The short extracellular regions contain characteristic cysteine residues responsible for dimerization in the mammalian subunits. According to Southern blot analysis, Xenopus laevis may possess two non-allelic genes for each subunit. Northern blots revealed FcRgamma transcripts of two sizes differentially expressed in thymus, spleen, intestine, liver and kidney. TCRzeta mRNA was predominantly expressed in the thymus and spleen. These data indicate that the amphibian immune system employs activating receptor complexes arranged in a mammalian-like way.