ABSTRACT. Freeze-fracture replicas of the cisternal membranes of dictyosomes, large vesicle (LV) membranes and plasma membranes in the green alga, Micrasterias crux-melitensis, were examined by electron microscopy. Intramembrane particles were evenly distributed in the cisternal membranes in resting cells. In growing cells, the particle density decreased from the centre to the periphery of the cisternal membranes and was greatly diminished at the vesiculated parts. The particle density in LV membranes was about twenty percent that in cisternal membranes, but equivalent generally in the plasma membranes of growing daughter semicells. The possibility of the direct supply of LV membranes to the plasma membrane is discussed. In the plasma membranes of fully grown cells, rosette complexes with a hexagonal array were observed. Each rosette consisted of six particles. These complexes were assumed to correlate with microfibril formation in the cell wall.Dictyosomes show the conspicuous structural changes during cell growth in Micrasterias crux-melitensis (23). This structural change largely consists of the formation of large vesicles (LVs) at the margins of the dictyosomal cisternae. The LVs, in turn, may contribute to the formation of primary walls, adding their contents exocytotically to growing cell walls (22,24). Thus, the series of movements and transformations of dictyosomes and their derivative vesicles are closely related to cell wall synthesis. The behavior of dictyosomes and LVs during cell growth has been extensively studied by electron microscopy of thin sections (23). However, these sections provide little information about the particles embedded in membranes. Recently, the structural relationship between cell wall microfibrils and plasma membrane particles has been pointed out using the freeze-fracture technique (10,14,16,18,19). In our study, intramembrane particles of dictyosomes, LVs and plasma membranes were examined by the freeze-fracture technique in order to gain further insight into cell wall synthesis and related questions.