The functional classification of ESTs from Puccinia striiformis f. sp. tritici -wheat compatible interaction with significant similarities to plant genes in the GenBank database. The homologies of unigenes to plant genes were classified into 13 functional categories based on their cellular roles. About 26% of the ESTs showed significant homology to enzymes involved in energy and metabolism, representing the largest category. 18% ESTs showed homologies to genes of unknown function and hypothetic proteins in the Genbank database.