Aromatic and aliphatic hydroxylations as well as U-demethylations occurred during metabolism of aromatic substrates by a range of fungi. para-Hydroxylation of monosubstituted aryl rings by fungi proceeded with migration and retention of deuterium label (NIH shift) and appeared to involve onlyarene 3.4-oxide intermediates, in common with plant and animal metabolism. The possibility of contributions from arene 1.2-or 2.3-oxide intermediates during orrho-hydroxylation is examined in the light of the results of oxygen-18 incorporation, the lack of deuterium-hydrogen primary kinetic isotope effects, and the wide range of deuterium migration and retention values. From these last experimental data no evidence for an arene oxide isomerase enzyme w a s found. istry, Queen'