FE65 is expressed predominantly in the brain and interacts with the C-terminal domain of [beta]-amyloid precursor protein (APP). We examined hippocampus-dependent memory and in vivo long-term potentiation (LTP) at the CA1 synapses with isoform-specific FE65 knockout (p97FE65[superscript -/-]) mice. When examined using the Morris water maze, p97FE65[superscript -/-] mice were impaired for the hidden platform task but showed normal performance in the probe test. To further discriminate the role of FE65 in acquisition and memory consolidation, we examined p97FE65[superscript -/-] mice with temporal dissociative passive avoidance (TDPA) and contextual fear conditioning (CFC). p97FE65[superscript -/-] mice showed impaired short-term memory for both TDPA and CFC when tested 10 min after training. After multiple TDPA training sessions, the crossover latency of some p97FE65[superscript -/-] mice reached the cutoff value, but it significantly decayed in 8 d. At the Schaffer collateral-CA1 synapses, p97FE65[superscript -/-] mice showed defective early-phase LTP (E-LTP). These results demonstrate novel roles of FE65 in synaptic plasticity, acquisition, and retention for certain forms of memory formation.