Social interaction is a complex behavior essential for many species and is impaired in major neuropsychiatric disorders. Pharmacological studies have implicated certain neurotransmitter systems in social behavior, but circuit-level understanding of endogenous neural activity during social interaction is lacking. We therefore developed and applied a new methodology, termed fiber photometry, to optically record natural neural activity in genetically and connectivity-defined projections to elucidate the real-time role of specified pathways in mammalian behavior. Fiber photometry revealed that activity dynamics of a ventral tegmental area (VTA)-to-nucleus accumbens (NAc) projection could encode and predict key features of social, but not novel object, interaction. Consistent with this observation, optogenetic control of cells specifically contributing to this projection was sufficient to modulate social behavior, which was mediated by type 1 dopamine receptor signaling downstream in the NAc. Direct observation of deep projection-specific activity in this way captures a fundamental and previously inaccessible dimension of mammalian circuit dynamics. Display omitted •Fiber photometry enables recording of neural projection activity in behaving mice•VTA-NAc projection activity encodes and predicts social interaction•Optogenetic control of VTA DA neurons bidirectionally modulates social behavior•Elevating VTA-NAc activity and NAc D1R signaling increases social behavior A new method called fiber photometry allows recording of the natural neuronal activity of neuronal projections in behaving mice. Such recordings in mice partaking in social interactions reveal that the dynamics of a neural projection between the ventral tegmental area (VTA) and nucleus accumbens (NAc) encode and predict key features of social behavior.