Stress relaxation in shear has been measured at 25°C for a polyisobutylene sample with viscosity average molecular weight 7.7×105, at shear strains γ from 0.2 to 0.5, over a time scale from 102 to 104.5 s. In this range, the relaxation modulus G(γ;t) could be expressed by the product G(0;t)h(γ); at γ=0.5, h=0.76. Simultaneous measurements of the differential storage and loss shear moduli, G′(ω,γ;t) and G″(ω,γ;t) were made throughout the relaxation process by intermittently superposing small oscillating deformations with a maximum additional strain of 0.01, at a frequency of about 0.33 Hz, which falls in the middle of the plateau zone of the polymer. For γ=0.2, G′ and G″ remained unchanged from their values at zero static strain, G′(ω,0) and G″(ω,0) respectively, confirming that the density of entanglements (or topological obstacles, or temporary network junctions), remains constant throughout the relaxation process at small strains. At higher strains, G″ was nearly constant, but G′ was somewhat smaller at the first measurement after imposition of static strain and slowly recovered to its original zero‐strain value. The behavior could be qualitatively interpreted by the Doi theory which includes both an equilibration process characterized by a time τe and a disengagement process characterized by a time τd; the decrease in G′ would correspond to loss of entanglements by the contraction of stretched molecules during equilibration and the subsequent increase to reestablishment of new entanglements by reptation during disengagement.