Crystalline cell inclusions were observed in 45 species of Cladophorophyceae. The crystals can be classified into eight morphological types, including needle-shaped, prismatic, octahedral, tetrahedral, cubic and globular, and they were found to occur in clusters or as single crystals. In addition to the different morphological types, the crystals are characterized by different chemical compositions. Chemical tests distinguished the crystals as being composed of calcium oxalate, calcium carbonate or proteins. The diversity of crystal types raises the possibility that these structures have systematic value. The occurrence of crystalline structures is compared with previously published phylogenies of the Cladophorophyceae. Some types of crystals were found to be genus- or species-specific, whereas other types occurred in distantly related groups. The crystalline cell inclusions can be useful diagnostic characters. For example, Cladophoropsis sundanensis and Cladophora coelothrix are distantly related but have similar thallus architecture, and they can be distinguished from one another by the presence or absence of crystals.