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Highlights:• Larval zebrafish raised in isolation show enhanced social avoidance reactions • Enhanced avoidance is composed of increased avoidance distances and usage of high acceleration escape swims• The lateral line sensory organ is necessary and sufficient for the increased usage of high acceleration escape swims Summary Social experiences greatly define successive social behavior. Lack of such experiences, especially during critical phases of development, can severely impede the ability to behave adequately in social contexts. To date it is not well characterized how early-life social isolation leads to social deficits and impacts development. In many model species, it is challenging to fully control social experiences, because they depend on parental care. Moreover, complex social behaviors involve multiple sensory modalities, contexts, and actions. Hence, when studying social isolation effects, it is particularly important to parse apart social deficits from general developmental effects, such as abnormal motor learning. Here, we characterized how social experiences during early development of zebrafish larvae modulate their social behavior, at one week of age, when social avoidance reactions can be measured as discrete swim events. We show that raising larvae in social isolation leads to enhanced social avoidance, in terms of reaction distance and reaction strength. Specifically, larvae raised in isolation use a high-acceleration escape swim bout, the short latency C-start, more frequently during social interactions. These behavioral differences are absent in non-social contexts. By ablating the lateral line and presenting the fish with local water vibrations, we show that lateral line inputs are both necessary and sufficient to drive enhanced social avoidance reactions. Taken together, our results show that social experience during development is a critical factor in shaping mechanosensory avoidance reactions in larval zebrafish.An important aspect of behavior is that it is adaptive and can change according to previous 2 outcomes of the actions of the self or others. During early development, as the nervous sys-3 tem builds and refines its connections, an animal is particularly sensitive to external sensory 4 input, or lack thereof (e.g. reviewed by Andersen [2003]). Social experiences during early 5 development can have long lasting effects on social and other behaviors, as shown by the dev-6