Abstract 1. 1. Calmodulin was purified from the anterior byssal retractor muscle (ABRM) of a mollusc Mytilus edulis. 2. 2. Ca 2+-induced conformational changes in the ABRM calmodulin could be demonstrated by polyacrylamide gel electrophoresis, by u.v. absorption spectrum and by circular dichroic spectrum. 3. 3. The amino acid composition of the ABRM calmodulin closely resembled that of other invertebrate calmodulins. 4. 4. The ABRM calmodulin was less effective in activating rat brain phosphodiesterase than vertebrate calmodulins.