Memories are not immutable; instead, they can be dynamically modified over time. In contextual fear conditioning, animals learn an association between a neutral context and an aversive event such as foot shocks. One day after conditioning, mice freeze significantly in the conditioned context but not in a novel context. With the passage of time, however, mice show freezing with an equivalent amount in both contexts, indicating that the contextual memory is generalized. This raises a fundamental question why the same sensory inputs elicit distinct behavioral expression of memory. To capture the dynamic changes in neuronal ensembles, we used a transgenic system in mice that allows us to examine the spatial patterns of active neurons at two different time points in a single mouse brain with a single cellular resolution. In this system, tetracycline-dependent transactivator (tTA) is expressed when the c-fos promoter, one of the immediate-early genes (IEGs) whose expression is rapidly and transiently induced in response to neuronal activities, is activated. In the absence of doxycycline, the tTA drives the tetO promoter linked tau-lacZ expression selectively in those neurons activated by the behaviorally relevant events, and the expression maintains for several weeks because of a positive feedback loop system. Using this transgenic system, we labeled neurons activated during fear conditioned-learning with tau-lacZ, and neurons activated during recall test with endogenous zif, one of the IEGs. By analyzing the proportion of neurons reactivated during memory retrieval, we found that neuronal ensemble activities were stable in the sensory cortex while they were dynamically changed in the hippocampus over time. Our results suggest that activity changes in the hippocampal neuronal ensembles might account for the generalized contextual fear memory.