The calcium/calmodulin-dependent kinase type II (CaMKII) holoenzyme of the forebrain predominantly consists of heteromeric complexes of the beta CaMKII and alpha CaMKII isoforms. Yet, in contrast to alpha CaMKII, the role of beta CaMKII in hippocampal synaptic plasticity and learning has not been investigated. Here, we compare two targeted Camk2b mouse mutants to study the role of beta CaMKII in hippocampal function. Using a Camk2b(-/-) mutant, in which beta CaMKII is absent, we show that both hippocampal-dependent learning and Schaffer collateral-CA1 long-term potentiation (LTP) are highly dependent upon the presence of beta CaMKII. We further show that beta CaMKII is required for proper targeting of alpha CaMKII to the synapse, indicating that beta CaMKII regulates the distribution of alpha CaMKII between the synaptic pool and the adjacent dendritic shaft. In contrast, localization of alpha CaMKII, hippocampal synaptic plasticity and learning were unaffected in the Camk2b(A303R) mutant, in which the calcium/calmodulin-dependent activation of beta CaMKII is prevented, while the F-actin binding and bundling property is preserved. This indicates that the calcium/calmodulin-dependent kinase activity of beta CaMKII is fully dispensable for hippocampal learning, LTP, and targeting of alpha CaMKII, but implies a critical role for the F-actin binding and bundling properties of beta CaMKII in synaptic function. Together, our data provide compelling support for a model of CaMKII function in which alpha CaMKII and beta CaMKII act in concert, but with distinct functions, to regulate hippocampal synaptic plasticity and learning.