Time resolved fluorescence quenching measurements with pyrene as probe are employed to determine aggregation numbers for hexadecyltrimethylammonium acetate (CTAOAc) and chloride (CTACl) micelles as a function of the concentration of detergent and added common-counterion salt. The aggregation numbers, N-A, of CTACl are roughly twice those of CTAOAc micelles at equivalent concentrations of detergent and salt, consistent with the known relative counterion binding affinity (chloride much greater than acetate). For both detergents, the increase of N-A with the net concentration of counterions in the intermicellar aqueous phase ([Y-aq]) follows the relationship found previously for anionic micelles: log N-A = log N-o + gamma log[Y-aq], where gamma and N-o are constants. However, compared to anionic micelles, for which gamma = 0.2-0.25, spherical micelles of both CTAOAc and CTACl exhibit less pronounced growth, with gamma = 0.1.