Closterium cells are crescent-shaped or elongate and lack spines. Some are quite straight and needle-like, while others are much broader with curved ends. The ends of the cell are usually tapered and may be pointed or rounded. Each semicell has a single axial, ridged chloroplast with at least one pyrenoid. Occasionally there are two chloroplasts per semicell. The nucleus is located in the center of the cell between the chloroplasts.

Terminal vacuoles at the cell tips hold vibrating crystals of barium or calcium sulfate, the function of which are unknown. Brownian motion causes these microscopic particles to move erratically due to the impacts of collisions with the surrounding liquid molecules in which they are suspended.

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