Aspergillus terreus, a moderately selenium-tolerant fungus, metabolized Se-selenite into several protein seleno-amino acids: selenomethionine and selenocysteine, as well as, nonprotein seleno-amino acids, selenocystathionine, and y-glutamyl selenomethyl selenocysteine. The results indicate the failure of the fungus to discriminate between sulphur and selenium. Selenium was also incorporated into several proteins of different molecular weights, mostly of low molecular weight proteins. Labeled studies showed the presence of high levels of selenomethionine and selenocysteine in the protein hydrolysate. The actual incorporation of protein seleno-amino acids into the fungal protein was proven. The results demonstrated a finding that detracts from previous held views.