Adult hippocampal neurogenesis has been linked to learning but details of the relationship between neuronal production and memory formation remain unknown. Using low dose irradiation to inhibit adult hippocampal neurogenesis we show that new neurons aged 4–28 days old at the time of training are required for long-term memory in a spatial version of the water maze. This effect of irradiation was specific since long-term memory for a visibly cued platform remained intact. Furthermore, irradiation just before or after water maze training had no effect on learning or long-term memory. Relationships between learning and new neuron survival, as well as proliferation, were investigated but found non-significant. These results suggest a new role for adult neurogenesis in the formation and/or consolidation of long-term, hippocampus-dependent, spatial memories.