Cell recognition molecules are involved in nervous system development and participate in synaptic plasticity in the adult brain. The close homolog of L1 (CHL1), a recently identified member of the L1 family of cell adhesion molecules, is expressed by neurons and glia in the central nervous system and by Schwann cells in the peripheral nervous system in a pattern overlapping, but distinct from, the other members of the L1 family. In humans, CHL1 (also referred to as CALL) is a candidate gene for 3p- syndrome-associated mental impairment. In the present study, we generated and analyzed CHL1-deficient mice. At the morphological level, these mice showed alterations of hippocampal mossy fiber organization and of olfactory axon projections. Expression of the mRNA of the synapse-specific neural cell adhesion molecule 180 isoform was upregulated in adult CHL1-deficient mice, but the mRNA levels of several other recognition molecules were not changed. The behavior of CHL1-deficient mice in the open field, the elevated plus maze, and the Morris water maze indicated that the mutant animals reacted differently to their environment. Our data show that the permanent absence of CHL1 results in misguided axonal projections and aberrant axonal connectivity and alters the exploratory behavior in novel environments, suggesting deficits in information processing in CHL1-deficient mice.