A cDNA clone encoding a 497 amino acid protein 75% similar to most Src proteins has been isolated from the planarian Dugesia (Girardia) tigrina (Platyhelminthes; Turbellaria) by PCR followed by screening procedures. This gene product has been designated Spk-1 as it is the first Src-related kinase isolated in a planarian. The predicted amino acid sequence of Spk-1 suggest that it is anchored to the plasma membrane and that it interacts with other phosphotyrosine proteins. Spk-1 is expressed in both intact and regenerating organisms as an mRNA transcript of about 1.9 kb. Planarians, which conserve most features of the common ancestor to protostomian and deuterostomian phyla, are the most primitive triploblastic organisms from which a protein tyrosine kinase gene product has been isolated. The presence of this gene product in such a primitive organism, and its presumed role, are discussed.