Plasmalemmasomes were noted in 6-week-old sporeling cells of the brown alga Petalonia debilis (Scytosiphonales). These membranous bodies had formed in pockets between the plasmalemma and cell wall. They consisted of a complex series of vesicles, tubules, and flattened cisternae, each of which was bounded by a unit membrane similar to the plasmalemma. Continuity was observed between the plasma membrane and some membranous structures, as they seemed to be forming by the processes of invagination and evagination.