Diverse glycoproteins of cell surfaces and extracellular matrices operationally termed 'adhesion molecules' are important in the specification of cell interactions during development, maintenance and regeneration of the nervous system^1,2. These adhesion molecules have distinct functions involving different cells at different developmental stages, but may cooperate when expressed together^3–6. Families of adhesion molecules which share common carbohydrate domains do exist, despite the structural and functional diversity of these glycoproteins^7,8,9. These include the Ca²⁺-independent neural adhesion molecules: N-CAM, myelin associated glycoprotein (MAG)¹⁰ and L1. L1 is involved in neuron-neuron adhesion⁶, neurite fasciculation¹¹, outgrowth of neurites¹², cerebellar granule cell migration¹³, neurite outgrowth on Schwann cells¹⁴ and interactions among epithelial cells of intestinal crypts¹⁵. We show here that in addition to sharing carbohydrate epitopes with N-CAM and MAG, L1 is also a member of the immunoglobulin superfamily. It contains six C2 domains and also shares three type III domains with the extracellular matrix adhesion molecule fibronectin.