The nerve terminal and the postsynaptic receptor-containing membranes of the electric organ are both linked to the basal lamina that runs between them. We have identified an extracellular matrix protein whose physical properties suggest it anchors the nerve terminal to the basal lamina. The protein was identified because it shares an epitope with a proteoglycan component of electric organ synaptic vesicles. It too behaves like a proteoglycan. It is solubilized with difficulty from extracellular matrix fractions, elutes from DEAE Sephacel at pH 4.9 only at high ionic strength, and binds to a laminin affinity column from which it can be eluted with heparin. Under denaturing conditions it sediments rapidly and has a large excluded volume although it can be included in Sephacryl S-1000 columns. This large, highly charged extracellular matrix molecule can be readily reconstituted into liposomes consistent with the presence of a hydrophobic tail. By immunoelectron microscopy the antigen is found both in synaptic vesicles and on the plasma membrane of the nerve terminal. Since this is the first protein described that links the nerve terminal membrane to the extracellular matrix, we propose calling it terminal anchorage protein one (TAP-1).