Polymer gels swollen in a good solvent and subjected to a uniaxial deformation have been studied by small-angle neutron scattering(SANS). Tn all the experiments, the solvent is deuterated; this is equivalent to deuteration of the entire network. The gels were studied at two degrees of swelling, Q : Q = II, which is very close to the preparation conditions (Q(prep) = 10), and Q = 23, which is very close to the equilibrium degree of swelling in pure solvent, Q(e) = 24. For each degree of swelling several elongation ratios ranging from 1.0 to 1.8 are investigated. For a fixed polymer concentration, it is shown that the scattered intensity at small angles in the direction parallel to the elongation axis increases with increasing deformation while it decreases in the perpendicular direction. Consequently, the isointensity curves mapped on a bidimensional detector are 8-shaped with a major axis parallel to the elongation direction. Such anomalous scattering contours, which cannot be explained by the classical theories of network deformation, are called "butterfly patterns". The experimental results are compared with the predictions of three recent models that account for butterfly isointensity curves in a SANS experiment of swollen elongated gels.