Understanding animal behavior and development requires visualization and analysis of their synaptic connectivity， but existing methods are laborious or may not depend on trans-synaptic interactions. Here we describe a transgenic approach for in vivo labeling of specific connections in Caenorhabditis elegans， which we term iBLINC. The method is based on BLINC (Biotin Labeling of INtercellular Contacts) and involves trans-synaptic enzymatic transfer of biotin by the Escherichia coli biotin ligase BirA onto an acceptor peptide. A BirA fusion with the presynaptic cell adhesion molecule NRX-1/neurexin is expressed presynaptically， whereas a fusion between the acceptor peptide and the postsynaptic protein NLG-1/neuroligin is expressed postsynaptically. The biotinylated acceptor peptide::NLG-1/neuroligin fusion is detected by a monomeric streptavidin::fluorescent protein fusion transgenically secreted into the extracellular space. Physical contact between neurons is insufficient to create a fluorescent signal， suggesting that synapse formation is required. The labeling approach appears to capture the directionality of synaptic connections， and quantitative analyses of synapse patterns display excellent concordance with electron micrograph reconstructions. Experiments using photoconvertible fluorescent proteins suggest that the method can be utilized for studies of protein dynamics at the synapse. Applying this technique， we find connectivity patterns of defined connections to vary across a population of wild-type animals. In aging animals， specific segments of synaptic connections are more susceptible to decline than others， consistent with dedicated mechanisms of synaptic maintenance. Collectively， we have developed an enzyme-based， trans-synaptic labeling method that allows high-resolution analyses of synaptic connectivity as well as protein dynamics at specific synapses of live animals.