Calmodulin, a calcium-binding protein with no known enzymatic activity but multiple, in vitro effector activities, has been purified to apparent homogeneity from the unicellular green alga Chlamydomonas reinhardtii and compared to calmodulin from vertebrates and higher plants. Chlamydomonas calmodulin was characterized in terms of electrophoretic mobility, amino acid composition, limited amino acid sequence analysis, immunoreactivity, and phosphodiesterase activation. Chlamydomonas calmodulin has two histidine residues similar to calmodulin from the protozoan Tetrahymena. However, unlike the protozoan calmodulin, only one of the histidinyl residues of Chlamydomonas calmodulin is found in the COOH-terminal third of the molecule. Chlamydomonas calmodulin lacks trimethyllysine but does have a lysine residue at the amino acid sequence position corresponding to the trimethyllysine residue in bovine brain and spinach calmodulins. The lack of this post-translational modification does not prevent Chlamydomonas calmodulin from quantitatively activating bovine brain phosphodiesterase. These studies also demonstrate that this unique calmodulin from a phylogenetically earlier eukaryote may be as similar to vertebrate calmodulin as it is to higher plant calmodulins, and suggest that Chlamydomonas calmodulin may more closely approximate the characteristics of a putative precursor of the calmodulin family than any calmodulin characterized to date.