Volatile organic compound (VOC) production from an endophytic fungus was quantified at four oxygen concentrations (0, 1, 13 and 21 %) throughout culture growth phases. The filamentous fungus, a Nodulisporium sp. (designated TI-13), was grown in a solid-state reactor with an agricultural byproduct, beet pulp, as the solid substrate. The VOCs, with potential applications as biofuels, natural flavour compounds and bioactive mixtures, were measured with a recently introduced platinum catalyst and proton transfer reaction mass spectrometry quantification system. The highest-specific production rates of carbon number four and higher VOCs were observed under microaerophilic conditions, which is the expected environment within the plant host. Specific production rates of two ester compounds increased by at least 19 times under microaerophilic conditions compared with those under any other oxygen concentration studied. Total VOC production, including small molecules such as ethanol and acetaldehyde, increased by 23 times when compared between aerobic and anoxic conditions, predominately due to increased production of ethanol. Additionally, total specific production for all 21 compounds quantified was highest under reduced oxygen conditions.