SUMMARYHypnospores, a type of sculptured‐walled resting spore formed by some zoosporic chlorophycean and chrysophycean algae, have been viewed at the ultrastructural level using thin section, SEM, and freeze‐etch techniques. The “spiny” wall of the hypnospore, as seen in the light microscope, appears as a raised, scalloped surface at the ultrastructural level. The PAS‐positive wall is composed of 2 distinct layers. Prior to the formation of the wall, vacuoles form in the cytoplasm, lipid deposits are detectable, and typical cytoplasmic organelles apparently degenerate. The entire process is completed in less t