Tyrosine aminotransferase (TAT, EC 2.6.1.5) from the kinetoplastid, Crithidia fasciculata, was purified over 2000 fold to electrophoretic homogeneity. The native form of the enzyme had a molecular weight of approximately 100,000, whereas under denaturing conditions it produced two polypeptides of approximately 50,000 and 48,000, respectively. Absence of a reaction with the periodic acid-Schiff stain suggested that the crithidial enzyme was not a glycoprotein. It was relatively stable and remained active over a wide range of pH and temperature. It exhibited a broad substrate specificity and was able to utilize L-tyrosine, L-tryptophan, and L-phenylalanine as amino donors. Antiserum produced against partially purified crithidial tyrosine aminotransferase failed to inhibit the enzymatic activity. The same antiserum cross-reacted with a soluble extract from Trypanosoma brucei gambiense, but not with that from normal mouse liver, confirming evolutionary conservatism between the two protozoa.