AbstractThe organic nitrogen requirements of six thermophilic fungi were examined in liquid shake cultures at 45 °C. The fungi used wereMucor pusillusLINDT,Humicola lanuginosa(GRIFFON and MAUBLANC) BUNCE,Thermoascus aurantiacusMIEHEsensuAPINIS,Thermoascus crustaceus(APINIS and CHESTERS) STOLK,Chaetomium thermophile var. coprophileCOONEY and EMERSON, andChaetomium thermophile var. dissitumCOONEY and EMERSON. The organic nitrogen sources tested were casein hydrolysate (vitamin‐free), L‐proline, L‐glutamic acid, DL‐serine, L‐alanine, DL‐valine, L‐threonine, L‐arginine, DL‐aspartic acid, L‐asparagine, DL‐glutamine, glycine, L‐leucine, DL‐isoleucine, L‐lysine, DL‐methionine, DL‐phenylalanine, and L‐histidine. Growth was determined by the mycelial dry weight method. All the organisms grew well on casein hydrolysate.M. pusillusgrew well on all the single amino acids, whileH. lanuginosagrew well on all, but six of them.T. aurantiacusandT. crustaceusgave a fair yield of mycelium on about 50% of the single amino acids tested, while the other 50% served as poor sources of nitrogen. All the single amino acids were poorly utilized byC. thermophile var. coproph