We have observed two distinct types of spatially modulated structures (on nanometer length scales) on the surface of Langmuir-Blodgett films of fatty acid salts using atomic force microscopy. Both involve periodic height modulation of the crystalline molecular lattice. The first type, seen in films containing Ba, Ca, and Mg, is commensurate with the molecular lattice and can be described as a regular series of packing defects that are consistent with the close-packing requirements of the alkyl. tail groups. The other type, seen in films containing Cd and Mn, is not always commensurate and is less perfectly correlated than the first type. We suggest a correspondence between these two types of modulations and the two predicted by a new toy model which incorporates the competition between the in-plane density preferred by the headgroups in conjunction with the cation and the close-packing requirements of all-trans crystalline alkyl chains.