We analyzed the freezing point depression and pNa measurements for aqueous solutions of sodium fusidate. At concentrations lower than 0.011 mol kg(-1), sodium fusidate behaves as a strong 1:1 electrolyte. At higher concentrations, sodium fusidate self-aggregates. To analyze the results two hypotheses on the monomer concentration are presented and discussed. The first one accepts that the monomer concentration, C(A), is constant and equal to 0.023 mol kg(-1). This concentration corresponds to a break point in the plot of the freezing point depression vs total sodium fusidate concentration, C(A)t. The second hypothesis accepts that C(A) increases with C(A)t following a leveling-off curve. Measurements of hydrodynamic radii and comparison with similar systems, such as sodium taurocholate, strongly support the second hypothesis. The results indicate that at concentrations lower than 0.08 mol kg(-1) the aggregation number increases from 2 to 3. Above this concentration, both the aggregation number and the fraction of bound counterions remain constant, with average values of 3.13 +/- 0.10 and 0.31 +/- 0.05, respectively. Such results indicate that for trimers only one Na+ counterion is involved per aggregate. We propose that this counterion shields the repulsion between the two nearest carboxylate groups which, according to a disklike model in which the monomers are packed with that group alternatively oriented up and down, should hold together. Values for the formation equilibrium constant of aggregates are also calculated. Its dependence with the aggregation number allows the determination of the reversible transfer of a free surfactant ion together with the associated counterions from the bulk solution to the aggregate, the resulting value being w(0) = -4.2k(B)T.