Diplosphaera Bialosuknia emend. Vischer (Chlorophyta; Trebouxiophyceae) is a very small green alga commonly found as a photobiont in lichens and free–living in soil. Cells of Diplosphaera are spherical or nearly so and often grouped in short chains. There is essentially no morphological variation among the strains of the single recognized species of this genus, D. chodatii. In our study of the soil algal communities of Warren Prairie Natural Area in southeast Arkansas, USA, we isolated several strains that matched the morphological characteristics of the genus Diplosphaera from the biotic crust of a sodic–saline slick. Phylogenetic analysis of DNA sequences from the ribosomal RNA internal transcribed spacer regions (ITS) and the plastid rbcL gene indicated that the Warren Prairie strains are a new species. This conclusion was also supported by analysis of the secondary structure of the ITS2. All of our results support the description of the new species, Diplosphaera sundellii. Key words: Chlorophyta, Diplosphaera, new species, phylogeny, sodic–saline soil, Trebouxiophyceae.