We experimentally investigated the effects of airflow rate on the specific growth rates of a blue-green algae, Spirulina platensis, and a green algae, Chlorella pyrenoidosa. For cultivation of C. pyrenoidosa, the specific growth rate was constant when the airflow rate was greater than 5 mL (.) s(-1). For cultivation of S. platensis, the specific growth rate had a maximum value when the airflow rate was 6 mL (.) s(-1). We found that an optimal flow rate existed for cultivation of S. platensis and that excess aeration tended to decrease its specific growth rate.