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Ammopiptanthus mongolicus is an evergreen xerophytic shrub in the legume family, Fabaceae, growing in Mid-Asian desert regions. Previously, we reported the isolation and characterization of ESTs from A. mongolicus for a large number of cold and drought-responsive gene sequences. Based on this, we selected a calmodulin (CaM) gene (designated AmCaM) for functional characterization. Bioinformatic analysis revealed that the full-length ORF of AmCaM was 450 bp and encoded 149 amino acids. Phylogenetic analysis revealed that the AmCaM protein showed high identity to homologs from Oryza sativa and Zea mays, and to the homolog from the leguminous species Vigna angularis. Quantitative RT-PCR analysis showed that AmCaM transcription was enhanced under both cold and heat stress conditions. The AmCaM gene was transformed into Escherichia coli cells for functional characterization. Heterologous expression of AmCaM improved freezing and heat tolerance of the transformant E. coli cells. These results indicate that AmCaM contributes to enhanced freezing and heat tolerance and suggest that it is a candidate gene for engineering of abiotic stress tolerance in crop, woody, or ornamental plants.