The apparent molal volumes and adiabatic compressibilities of aqueous solutions of the sodium salts of the amphiphilic anionic penicillins cloxacillin, dicloxacillin, and nafcillin have been determined from density and ultrasound velocity measurements at 298.15 K. Critical concentrations were obtained from both techniques, confirming the existence of a second critical concentration for dicloxacillin, as had been previously reported. Positive deviations of the concentration dependence of the apparent molal volumes of cloxacillin and dicloxacillin from that predicted by the Debye-Huckel limiting law in dilute solution provide evidence for limited association at concentrations below the first critical concentration. Isentropic apparent molal adiabatic compressibilities of the aggregates formed by the drugs, calculated by combining the ultrasound velocity and density data, were typical of those for a stacked aggregate.