Time-resolved fluorescence quenching has been used to determine the aggregation number, N, of a series of zwitterionic surfactants of the alpha-(dodecyldimethylammonio)lkane-omega-carboxylate type with the intercharge alkanediyl group being CH2, C3H6, C5H10, and C10H20 (referred to as C12N1CO2, C12N3CO2, C12N5CO2 and C12N10CO2, respectively), in order to investigate the dependence of N on the carbon number of the intercharge group. 1-(Dodecyldimethylammonio)propane-3-sulfonate (C12N3SO3) has also been investigated to establish the effect of the head group. The aggregation numbers have been found to depend very little on the surfactant concentration and salt content of the solution. An increase in temperature resulted in a decrease of N for C12N1CO2, C12N3CO2, C12N3SO3, and C12N5CO2, with the magnitude of the decrease becoming smaller in this sequence. The aggregation number of C12N10CO2 slightly increased with temperature. Irrespective of the surfactant concentration and NaCl content, the values of N at a given temperature for the various surfactants were in the sequence : C12N1CO2 > C12N3SO3 > C12N3CO2 > C12N5CO2 approximate to C12N10CO2. However, the values of N indicated that the micelles of these surfactants were spherical or spheroidal, under the experimental conditions used. The results are discussed on the basis of the variation of the conformation of the intercharge alkanediyl group with its carbon number.