In this paper, we studied the mechanism of the increase of firmness in strawberry fruit treated with 100% CO₂. Internal CO₂ concentration in treated fruit increased immediately after the treatment and maintained the high level during treatment. But, it decreased rapidly after release of the treatment and reached the same level of untreated ones. This treatment rapidly decreased the ratio of water soluble pectin content against total pectin content and increased the ratio of hexametaphosphate soluble pectin content during and after the treatment, but did not affect the ratio of hydrochloric acid soluble pectin to total pectin in the fruit. Pectin methyl esterase activity was higher in treated fruit than the one in untreated fruit during and after the treatment. The ratio of Ca content of NaCl-soluble fraction (contained pectin substances) to total Ca content was higher in the treated fruit during and after treatment than the one in untreated fruit. These results suggest that the increase of firmness of strawberry fruit by 100% CO₂ treatment may be due to the formation of intramolecular bridges in pectin, mediated by Ca²⁺