THE principal requirement for materials that exhibit second-harmonic generation-frequency-doubling of light-is that they have a non-centrosymmetric structure. For optically active organic materials, this can be achieved by the use of the Langmuir-Blodgett (LB) technique1,2 : amphiphilic molecules comprising a hydrophilic head (the chromophore) and a hydrophobic tail are aligned at an air-water interface, and the resulting ordered monolayer is transferred to an appropriate solid substrate. For practical applications, the non-centrosymmetric structure of the individual monolayers needs to be maintained across many such layers, but this has been achieved previously for relatively few optically active molecules3-5. Most amphiphiles pack centrosymmetrically, with the layers adopting a head-to-head and tail-to-tail ’Y-type’ structure6. To enforce a non-centrosymmetric structure, it becomes necessary to intersperse the optically active layers with another material7-12. Here we demonstrate an alternative strategy for achieving macroscopic non-centrosymmetric order in LB multilayers. We find that the addition of a second hydrophobic end-group to the hydrophilic chromophore reduces the tendency of the molecules to invert during deposition, and a head-to-tail ’Z-type’ structure can be readily obtained for films containing more than 100 layers.