Phosphorylation by protein tyrosine kinases is crucial to the control of growth and development of multicellular eukaryotes, including humans, and it also seems to play an important role in multicellular prokaryotes. A plant tyrosine-specific kinase has not been identified yet; hence, plants have been suggested to share with unicellular eukaryote yeast a tyrosine phosphorylation system where a limited number of stress proteins are tyrosyl-phosphorylated only by a few dual-specificity (serine/threonine and tyrosine) kinases. However, preliminary evidence obtained so far suggests that tyrosine phosphorylation in plants depends on the developmental conditions. Since sequencing of the genome of the model flowering plant Arabidopsis thaliana has been recently completed, we have performed a bioinformatic screening of the whole Arabidopsis proteome to identify a model complement of bona fide protein tyrosine kinases. In silico analyses suggest that < 4% of Arabidopsis kinases are tyrosine-specific kinases, whose gene expression has been assessed by a preliminary polymerase chain reaction screening of an Arabidopsis cDNA library. Finally, immunological evidence confirms that the number of Arabidopsis proteins specifically phosphorylated on tyrosine residues is much higher than in yeast.