Relaxation of stress in simple extension of polyisobutylene (viscosity‐average molecular weight2.3×106) has been measured at stretch ratios (λ) up to about 2. Data at 3, 25, and 50°C were reduced to 25°C. At any instant of reduced time, the dependence of stress on λ could be described by the Mooney‐Rivlin equation; the coefficientsC1andC2were obtained from two linear plotting procedures. At short times,C1decreased rapidly whileC2remained essentially constant. Two to three logarithmic decades of time later,C2decreased. Thus, the terminal zone of viscoelastic behavior corresponds solely to theC2contribution. This result confirms measurements by Arai and Niinomi (Kogyo Kagaku Zasshi,74, 2525 (1971)) on polyisoprene and styrene‐butadiene rubber and measurements by Noordermeer and Ferry (J. Polym. Sci., (1976), in press) on 1,2‐polybutadiene. Tentative interpretations in terms of motions of entanglements are discussed. Similar measurements on a sample of polyisobutylene which had been heated with vulcanizing ingredients and was very slightly crosslinked gave higher values ofC2but lower values ofC1.