Calmodulin mRNA has been partially purified from a total nucleic acid extract of the electroplax of Electrophorus electricus by oligo(dT)-cellulose chromatography and sucrose gradient centrifugation. A 9- to 10S fraction was determined to contain 39% calmodulin mRNA by translation in a reticulocyte lysate followed by immunoprecipitation with antibodies to calmodulin. Double-stranded cDNA was synthesized from the RNA fraction by using reverse transcriptase from avian myeloblastosis virus. The double-stranded cDNA was joined to pBR322 linearized by restriction endonuclease Pst I and used to transform Escherichia coli RRI. DNAs from 60 tetracycline-resistant cloned hybridized to [32P]cDNA synthesized from the partially purified calmodulin mRNA fraction. By direct DNA sequence analysis, one of these clones, pCM109, was shown to contain calmodulin-specific sequences corresponding to amino acid residues 93--148 of calmodulin or approximately 38% of the peptide-coding region of the calmodulin structural gene sequence. pCM109 was hybridized to DNA isolated from three vertebrate and one plant species by the procedure of Southern. Positive hybridization bands were noted regardless of the DNA source. These data suggest thaat calmodulin gene sequences are evolutionarily conserved, as has been shown to be the case for the primary amino acid sequence.