Measurements of ¹³C values were used as a diagnostic test for the possible occurrence of C₄ photosynthesis in 175 species of the Chenopodiaceae and 18 species of the genus Calligonum (Polygonaceae) from deserts of the Middle East and USSR. Eighty percent of the Chenopodiaceae (predominantly members of the genera Aellenia, Anabasis, Haloxylon, Salsola and Suaeda) and all species of the genus Calligonum showed C₄-like ¹³C values. Several features of these plants disclose some new facets of C₄ photosynthesis. Some of the Haloxylon and Calligonum species are trees or tall shrubs and in Middle Asia are dominant members of plant communities characterized by high biomass. Many C₄ species of the Chenopodiaceae and Polygonaceae in their natural, Middle and Central Asian desert habitats, experience temperatures far below the freezing point for a long period of the year. Several of these C₄ species are of considerable economic value.