Transmission electron microscopy images of the treelike fractal aggregates of CdS nanoparticles in amphiphilic oligomer (polymaleic acid with octadecanol ester) LB matrix were observed. The features of these aggregates were developed into the micrometer regime. By changing the ratio between carboxylic groups and hydrocarbon chains, as well as the surface pressure, different fractal CdS patterns could be obtained. These results lead us to propose the mechanism of the formation of branchlike patterns, very much analogous to two-dimensional irreversible fractal-growth models such as diffusion-limited aggregates (DLA). Weak hydrophobic and hydrophilic interaction for interfacial molecular recognition at the organic/inorganic interface had been proposed as providing a possible elucidation for the formation of some aspects of a self-similar fractal-like pattern. At a ratio of 7:1, the fractal dimension was calculated by the box-counting method, and the dominant value was 1.69 +/- 0.03, which was in good agreement with two-dimensional DLA.