Therefore, in this study, we conducted batch culture using the freshwater diatom Nitzschia palea with four levels of silicon concentration (5 to 30 mg/L) and investigated the effects of silicon concentration in the water environment on diatom growth and EPA production. As a result, there was no significant difference in the specific growth rate during the logarithmic growth phase, but a positive correlation was confirmed between population density and biomass (dry weight) production during the stationary phase and silicon concentration. In addition, the dry weight and shell length per cell were significantly different from those of the other systems at the lowest silicon concentration of 4.93 mg/L. EPA contents of population was significantly higher at a silicon concen tration of 29.6 mg/L than at a silicon concentration of 4.93 mg/L. EPA contents per cell was significantly higher in the silicon concentration of 4.93 mg/L than in the other systems. Therefore, it was suggested that diatoms with long shells tended to be distributed at low density in an environment with low silicon concentration, which deteriorated the food environment for consumers.