The ultrasonic absorption of sodium cholate, deoxycholate and dehydrocholate has been measured as a function of concentration in water, H 2O, 0.2 M NaCl, and methanol. The effect of pH has also been investigated. The results indicate that the observed excess absorption is due to the aggregation of the bile acid ions, the association being much stronger for deoxycholate than for cholate and negligible for dehydrocholate. The measurements of density confirm the ultrasonic absorption results. However, the excess ultrasonic absorption of bile salt solutions has features differing considerably from those found for solutions of classical detergents where a true micellization takes place. In particular the ultrasonic absorption relaxation spectra of bile salt solutions show evidence of a distribution of relaxation frequencies rather than a single relaxation frequency as found for ionic detergents. These results lead to the conclusion that the association takes place in the whole range of concentration and not at some critical micelle concentration, and that the distribution curve of aggregate sizes must be wide and shifting upward as the concentration is increased.