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We study the anomalous flux ratio which is observed in some four-image lenssystems, where the source lies close to a fold caustic. In this case two of theimages are close to the critical curve and their flux ratio should be equal tounity, instead in several cases the observed value differs significantly. Themost plausible solution is to invoke the presence of substructures, as forinstance predicted by the Cold Dark Matter scenario, located near the twoimages. In particular, we analyze the two fold lens systems PG1115+080 andB1555+375, for which there are not yet satisfactory models which explain theobserved anomalous flux ratios. We add to a smooth lens model, which reproduceswell the positions of the images but not the anomalous fluxes, one or twosubstructures described as singular isothermal spheres. For PG1115+080 weconsider a smooth model with the influence of the group of galaxies describedby a SIS and a substructure with mass $\sim 10^{5} M_{\odot}$ as well as asmooth model with an external shear and one substructure with mass $\sim 10^{8}M_{\odot}$ . For B1555+375 either a strong external shear or two substructureswith mass $\sim 10^{7} M_{\odot}$ reproduce the data quite well.

Substructures in lens galaxies: PG1115+080 and B1555+375, two fold configurations