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2′,3′‐cyclic nucleotide 3′‐phosphodiesterase (CNPase) is a quantitatively major enzyme in myelin, where it localizes to the non‐compact regions and is bound to the membrane surface. Although its catalytic activity in vitro has been characterized, the physiological function and in vivo substrate of CNPase remain unknown. Especially the N‐terminal domain has been poorly characterized; previously, we have shown it is involved in CNPase dimerization and RNA binding. Here, we show that purified CNPase binds to the calcium sensor protein calmodulin (CaM) in a calcium‐dependent manner; the binding site is in the N‐terminal domain of CNPase. CaM does not affect the phosphodiesterase activity of CNPase in vitro, nor does it influence polyadenylic acid binding. The colocalization of CNPase and CaM during Schwann cell myelination in culture was observed, and CaM antagonists induced the …