The interactions between dodecyl trimethylammonium bromide (C(12)TAB) and two samples of DNA with widely differing molecular weights have been studied using surface tension and neutron reflectometry. Neutron reflection data show that the surfactant and polymer are adsorbed together in a highly cooperative fashion over a 1000-fold change in surfactant concentration. Furthermore, the shorter DNA fragments adsorb with C(12)TAB as trilayers at higher surfactant concentrations, with overall layer thicknesses of 65-70 angstrom. The high molecular weight DNA, however, shows only approximate monolayer adsorption with thicknesses varying from 19 to 26 A over the entire range of C(12)TAB concentrations. The difference in behavior between the different samples is believed to be a result of the rigid double helical structure of DNA which makes the formation of bulk phase polymer/micelle aggregates much less favorable for the short fragments. The resulting increase in the critical aggregation concentration (CAC) then leads to the adsorption of additional surfactant/polymer complex to the underside of the initial stable surface active DNA/C(12)TAB complex. Comparison with previous results obtained for synthetic polyelectrolytes shows that DNA/C(12)TAB complexes are not capable of reducing the surface tensions to the extent that other mixtures such as the poly(styrene sulfonate)/C(12)TAB mixtures do. A possible reason for this is the high rigidity of DNA combined with the fact that its hydrophobic moieties are positioned within the double helix so that the external molecule is largely hydrophilic.