Small but significant amounts of silica occur in all brown algae and green Hydrodictyaceae examined. At least in the hydrodictyaceous Pediastrum and Tetraëdron, the silica predominates in the cell wall. This silica is largely isotropic, non-crystalline opal with a small amount of α-quartz. Occurrence of silica in brown algae constitutes yet another feature suggesting phylogenetic relationships with the Chrysophycophyta, and the siliceous cell walls of the Hydrodictyaceae facilitate an understanding of both the nature of organic substances present and the process of zoosporogenesis.