The cyclic AMP (cAMP)-response element binding protein (CREB) is an activity-dependent transcription factor that plays a role in synaptic plasticity and memory storage in Aplysia, Drosophila, and rodents. Mice with targeted deletions of two CREB isoforms (α and Δ; CREB αΔ mice) have been characterized on a mixed genetic background of C57BL/6 (B6) and 129/SvEv (129), as well as on a defined F1 hybrid of B6 and FVB/N, and these results suggest that the phenotype of CREB αΔ mice depends critically on genetic background. In an examination of the hypothesis that the role of CREB in learning and memory can be influenced by strain differences, we analyzed mice with the CREB αΔ mutation on an F1 hybrid background of B6 and 129 strains. CREB αΔ mice on this background had impaired short-term and long-term cued and contextual fear conditioning and normal spatial learning in the Morris water maze. Our results suggest that at least some aspects of hippocampal function are normal in CREB αΔ mice, and that CREB αΔ mice on the B6/129 F1 background have alterations in amygdala function. These studies underscore the importance of controlling for genetic background in the behavioral analysis of knockout and transgenic mice. Hippocampus 2002;12:18–26. © 2002 Wiley-Liss, Inc.