•Ultra-high resolution crystal structure of caprine βlg.•AUC and SAXS data confirm that caprine βlg is a dimer in solution.•AUC data (between 5 and 50μM) suggest that the protein is in a monomer–dimer self-association. β-Lactoglobulin (βlg) is the most abundant whey protein in the milks of ruminant animals. While bovine βlg has been subjected to a vast array of studies, little is known about the caprine ortholog. We present an ultra-high resolution crystal structure of caprine βlg complemented by analytical ultracentrifugation and small-angle X-ray scattering data. In both solution and crystalline states caprine βlg is dimeric (KD<5μM); however, our data suggest a flexible quaternary arrangement of subunits within the dimer. These structural findings will provide insight into relationships among structural, processing, nutritional and immunological characteristics that distinguish cow’s and goat’s milk. cBlg and cBlgbind by cosedimentation in solution (View interaction) bBlg and bBlgbind by cosedimentation in solution (View interaction) cBlg and cBlgbind by X ray scattering (View interaction) cBlg and cBlgbind by X-ray crystallography (View interaction)