Autophosphorylation of a calcium - calmodulin-dependent kinase II (alpha CaMKII) has been proposed to be the key event in memory storage. We tested this hypothesis with autophosphorylation-deficient mutant mice in hippocampus- and amygdala-dependent learning and memory tasks and found that the autophosphorylation of aCaMKII was required for rapid learning but was not essential for memory. We conclude that aCaMKII autophosphorylation contributes to single-trial learning but is dispensable for memory.