Fungal development is largely affected by many environmental factors. In a model filamentous fungus Aspergillus nidulans, asexual development is promoted by exposure of light, presence of salt and non-fermentable sugars. In other hand, sexual development is largely induced by absence of light, fermentable sugars and hypoxic condition. Also, some important genes including veA and nsdD play positive roles in activating sexual development. Here, we reported that the effect of high concentration of CO₂ on developmental decision in A. nidulans. When wild-type veA ⁺ strain was cultured in normal condition, sexual and asexual development occurred in balanced manner. However, high concentration of CO₂ (~5%) strongly activated sexual development and inhibited asexual development. Furthermore, this CO₂ effect was controlled by the veA or nsdD gene. High CO₂ culture of veA – or nsdD – mutant didn't activate sexual development, suggesting that the activation of sexual development induced by high CO₂ cannot overcome the genetic requirement of sexual development such as veA or nsdD. Since 5% CO₂ is an important condition for human pathogenic fungi for surviving and adapting in human body, this developmental pattern of A. nidulans affected by CO₂ concentration may provide interesting clues for comparative study with human fungal pathogens including Aspergillus fumigatus.