Abstract Casein micelles in skim milk crude opalescent layer were not fractionated by rate-zone ultracentrifugation because they failed to sediment sufficiently into the gradient. Gel filtration indicated that soluble casein in the ultracentrifugal supernatant from crude opalescent layer consisted of submicellar casein complexes larger than those in skim milk soluble casein. Inelastic light scattering indicated that the hydrodynamic radii of casein micelles in opalescent layer ranged from 100 to 200nm. Electron microscopy revealed that opalescent layer casein micelles are polydisperse and predominantly smaller than those in skim milk casein pellet (≤100nm diameter). Irregularly shaped particles, that may be lipoprotein fragments of fat globule membranes, were also observed. Casein micelles, pelleted from heated skim milk, exhibited intermicellar interaction and were not readily redispersed by grinding and stirring at 0 to 5C. The temperature dependent association-dissociation of soluble casein from opalescent layer casein micelles and the high concentration of heat-induced β-lactoglobulin-κ-casein complex in opalescent layer were further confirmed by zonal electrophoresis.