We studied roles of DN-cadherin, the Drosophila major neuronal cadherin, in neuronal connections in the visual system. In DN-cadherin mutants, axon terminals of a large subset of photoreceptor cells reached and associated with their target interneurons, but their characteristic spatial arrangement was disrupted as synaptogenesis proceeded. Although synapses were formed at contact sites between the axon terminals and target neurons, underlying cytoplasmic structures were not fully specialized at both pre- and postsynaptic terminals and synaptic vesicles appeared to accumulate at the presynapses. These results suggest that the cadherin adhesion system is required for interaction between pre- and postsynaptic terminals and for generation of the mature synaptic structures.