By analyzing uniaxial, planar, ellipsoidal, and equibiaxial extension data, a viscoelastic constitutive equation of the single integral form is derived for a polyisobutylene (PIB) melt. Within experimental limits, time and strain effects are separable. The strain dependence corresponds to a Mooney‐type combination of Finger and Cauchy tensors, modified by a damping function h(I1,I2), which depends on the strain invariants I1 and I2 of the Finger tensor. It is shown that the ratio of the second to the first normal stress difference in shear flow β can be extracted from planar extension experiments. For the PIB melt considered, β turns out to be negative and equal to β=−0.27, and to be independent of strain. The influence of β on the tensions in general extensional flow is discussed.