Technical Abstract: The entomopathogenic fungus Isaria fumosorosea is capable of dimorphic growth (hyphal or yeast-like) in submerged culture. In shake flask studies, we evaluated the impact of aeration on the mode of growth of I. fumosorosea. Using 250 mL baffled Erlenmeyer flasks, culture volumes of 50, 100, 150, and 200 mL of I. fumosorosea were grown in a shaker incubator at 350 rpm and 28 °C. Dissolved oxygen (DO) was continuously monitored using a non-invasive oxygen monitoring system. Culture volumes of 50 mL maintained DO concentrations above 10% throughout the 3 day growth period. Fifty mL cultures accumulated biomass and produced blastospores more rapidly than the other culture volumes tested and more than 55% of their biomass was blastospores. Dissolved oxygen was depleted in culture volumes of 100, 150, and 200 mL after a growth period of 20.5, 16.8 and 13.5 hours, respectively. In 100 mL cultures, DO was below 10% for 8 hours whereas DO in 150 and 200 mL cultures were exhausted (< 3%) for the remainder of culture growth resulting in significantly lower blastospore yields and increased hyphal growth. These studies were used to set oxygen concentration requirements (> 20% DO) for I. fumosorosea growth in a 100 L bioreactor resulting in comparable blastospore production with minimal hyphal growth. Maintaining higher DO levels resulted in increased blastospore production by cultures of I. fumosorosea grown on nitrogen sources (cottonseed meal and soy flour) that previously elicited excessive hyphal growth. These studies have shown that oxygen availability is essential for significant yeast-like growth by I. fumosorosea cultures and that continuous monitoring of oxygen concentrations in shake flask cultures can be a useful tool for establishing appropriate aeration conditions for use in bioreactors.