Summary: The cells of Navicula pelliculosa become invested in a gelatinous capsule when they cease to divide. This may occur when the culture medium becomes depleted of some necessary element, such as silicon, phosphorus, or nitrogen. A method for the extraction of the capsular fraction has been devised, which is based on its solubility in 20% (w/v) NaOH. It is a polyuronide, apparently consisting solely of glucuronic acid residues.