Organismal phenotypes frequently involve multiple organ systems. Histology is a powerful way to detect cellular and tissue phenotypes, but is largely descriptive and subjective. To determine how synchrotron-based X-ray micro-tomography (micro-CT) can yield 3-dimensional whole-organism images suitable for quantitative histological phenotyping, we scanned whole zebrafish, a small vertebrate model with diverse tissues, at ~1 micron voxel resolutions. Using micro-CT optimized for cellular characterization (histo-tomography), brain nuclei can be computationally segmented and assigned to brain regions. Shape and volume can be computed for populations of nuclei, motor neurons and red blood cells. Computed cell density revealed striking individual phenotypic variation. Unlike histology, histo-tomography allows the detection of phenotypes that require millimeter scale context in multiple planes. We expect the computational and visual insights into 3D tissue architecture provided by histo-tomography to be useful for reference atlases, hypothesis generation, comprehensive organismal screens, and diagnostics.