A magnetic rod interfacial shear rheometer was used to measure the properties of beta-casein adsorbed at the air/solution interface as a function of aging time. Over a wide range of concentrations (1 X 10(-6) to 2 x 10(-2) wt % beta-casein) the initial rheology of the adsorbed surface layer is dominated by a viscous response of the interface. For solutions in the range 5 x 10(-3) to 2 x 10(-2) Wt%, interfacial gelation is observed after similar to15 h of aging, long after the surface tension has stabilized. In particular, although both components of the complex interfacial shear modulus (i.e., the storage and loss moduli) gradually increase with aging time, the ratio of the loss to the storage modulus-the loss tangent-decreases and drops below unity. The frequency dependence of the shear modulus is consistent with sol-gel transitions observed in bulk systems and described within the context of percolation theory.