Micellar solutions of cetyltrimethylammonium bromide (CTAB) in water and of sodium dodecyl sulfate (SDS) in 0.22 m NaCl were investigated with field dependent H-2 nuclear magnetic relaxation. The full theoretical model for the molecular dynamics of aggregated surfactant molecules recently developed by Halle (J. Chem. Phys. 1991, 94, 3150.) was applied, making it possible to evaluate the relaxation data in terms of aggregate sizes and shapes. Average aggregation numbers were determined within a simple model for the polydispersity of the solutions. For both systems it was found that nonspherical aggregates were formed at concentrations well below the second cmc. Above the second cmc, the aggregates were found to be rod-shaped and the average aggregation number increased rapidly with the concentration.