By using folic acid-2-14C in the growth medium as the sole source of pteridine it was found that the kinetoplastid flagellate, Crithidia fasciculata, produced a total of 87 mμg of labeled biopterin and 23 μg of 2-amino-4-hydroxy-6-hydroxymethylpteridine from 2 mg of folic acid in 2 liters of medium. Eighty percent of the biopterin and 70% of the 6-hydroxymethylpteridine was isolated from the spent medium, the remainder from the cells. These results and the results whereby the total pteridine was supplied as a folic acid analog make it obvious that this flagellate does convert folate to biopterin and is incapable of de novo pteridine synthesis.