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Green algae are present in desert soils as components of microbiotic communities that also include cyanobacteria and other prokaryotes, lichens, non-lichenized fungi, invertebrates, and other photosynthetic eukaryotes such as diatoms, eustigmatophytes, and xanthophytes. The green algae that occur in crusts are morphologically simple unicells, packets of cells, or weak filaments, yet represent a diverse assemblage of taxa spanning the classes Chlorophyceae, Trebouxiophyceae, and Charophyceae. As part of an ongoing study of the biodiversity of microbiotic crust communities in the western United States and Northern Mexico, a large number of green algae were isolated and characterized morphologically and genetically. Phylogenetic analyses using ribosomal RNA gene sequence data have greatly aided our understanding of the diversity and evolution of desert green algae. Our results indicate that desert green algae evolved from aquatic green algae at least five independent times. In addition, the desert green algae are derived from freshwater, not marine, green ancestors. Some lineages of green algae have a high proportion of desert taxa, while other lineages thus far have no known desert representatives. Many of the isolates are likely to be new taxa. These taxa represent independent lineages of green plants that have evolved to inhabit desert environments. Because they are distinct from but phylogenetically related to embryophyte taxa, these other "land plants" can offer important biochemical and physiological comparisons to desert-dwelling embryophytes.