Understanding how distributed neuronal circuits integrate sensory information and generate behavior is a central goal of neuroscience. Yet, studying neuronal networks at single-cell resolution across the entire adult brain has been difficult in vertebrates due to their size and opacity. We address this challenge by introducing the fish Danionella translucida as a model organism to neuroscience. This teleost remains small and transparent even in adulthood, when neural circuits and behavior have matured. Despite its small brain, Danionella displays a rich set of complex behaviors, including courtship, shoaling, schooling and acoustic communication. To enable optical activity measurements and perturbations with genetically encoded tools, we established CRISPR/Cas9 genome editing and Tol2 transgenesis techniques. These features make Danionella translucida a promising model organism for the study of adult vertebrate brain function at single-cell resolution.