Following the earlier study of theβ- andϰ-casein micelle structure, we will now report results from theαs1-casein. Static and dynamic light scattering measurements were performed in a concentration range from 0.5 to 6.0 mg/ml atT=35 °C. A constant apparent molecular weight of 3.4×106 daltons was found over the whole range. The apparent radii of gyration and the diffusion coefficients also show no detectable concentration dependence. The ratio of the two radiiϱ≡R g /R H =2.78+0.21 is characteristic of extended rigid structures.R g is the radius of gyration andR H the hydrodynamic radius defined via the Stokes-Einstein relationship from the translational diffusion coefficient. This is in agreement with the analysis of the pronounced angular dependence of the scattered light, which leads to the conclusion thatαs1-casein forms very long worm-like micelles. The contour length of one cylinder was found to beL∼1600 nm and the chains appear to be composed of about 12 Kuhn segments. At higher concentrations, lateral aggregation proportional to the concentration is observed. Beyond the overlap concentrationc* the asymptotic scattering curve changes its shape, which is interpreted as the beginning of a reversible gelation.