Two hundred eighty-five fungi, including 100 basidiomycetes and 35 yeasts, 75 actinomycetes, and 40 bacteria were screened for their ability to convert 5-anilino-1,2,3,4-thiatriazole (AT) to 5-(p-hydroxyanilino)-1,2,3,4-thiatriazole (p-HT). Eleven cultures were found that formed p-HT, which was isolated and whose structure was determined. Aspergillus tamarii NRRL 3280 formed 8.6 g of p-HT/liter from 10 g of AT/liter (78.9% conversion) in shaken flasks and 4.57 g of p-HT/liter from 6 g of AT/liter (69.8% conversion) in 30-liter fermentors. Washed cells of A. tamarii NRRL 3280 also carried out this conversion. 5-(o-hydroxyanilino)-1,2,3,4-thiatriazole (o-HT) was identified as a second product formed by Aspergillus terreus NRRL 1960.