A quasi-crystalline lamellar lattice was observed in chloroplasts of the filamentous green alga Zygnema. The lattice does not appear in the cells until cultures are at the end of the log phase of growth. Pseudograna are also present and become more numerous towards the middle of the log phase. The three-dimensional lattice superficially resembles the configuration of cubic prolamellar bodies but is about 10 times larger and is entirely different in internal structure. The lattice is composed of one or two appressed thylakoids in a stroma matrix which is bounded on each side by a single thylakoid membrane. This multilayered sandwich of membranes and matrix occupies a position equivalent to the single membrane of a cubic prolamellar body.