AbstractThe nexus of the anterior byssus retractor muscle (ABRM) ofMytilushas been observed with freeze‐fracture techniques. Pits appear in the EF face and particles in the PF face, designating it as an A‐type nexus. Under control conditions (lanthanum‐free) the center‐to‐center spacing of pits and particles ranged from 14.0 nm to 15.5 nm. When lanthanum chloride (5 mM) was used in incubation media before fixation and glycerination, marked changes in the nexal membrane were observed. These changes involved close packing and long‐range ordering of pits in the EF face, as shown by optical diffraction. In the presence of lanthanum the center‐to‐center spacing of the pits was 11.7 nm, while the particles remained at a spacing of 15.0 nm. From this we conclude that specific agents can alter the array of pits and particles in a nexus