Stringent requirements on the dimensional stability of polymer films used as substrates for magnetic recording make it necessary to experimentally determine their anisotropic viscoelastic behavior. This paper deals with the measurement of the time-dependent, longitudinal elongation and lateral contraction of 50 × 900-mm biaxially oriented orthotropic polyethylene terephthalate (PET) strips. Two different methods are described for measuring longitudinal elongation, and a laser-scanning technique is used for measuring lateral contraction. Preliminary investigations are carried out to determine the static Poisson's ratio, to check the linearity of longitudinal creep with respect to load, and to investigate the validity of the time-temperature superposition hypothesis. In addition, tests are described in which longitudinal and lateral creep of the specimens are simultaneously measured under constant loads in temperature- and humidity-controlled environments. It is found that the Poisson's ratio has only a weak dependence on time, and, therefore, a good approximation may be obtained by treating the Poisson's ratio as independent of time.