An instrument has been constructed to allow simultaneous measurements of stress and birefringence in polymer samples to be made as a function of time and temperature. The stress is measured by attaching one end of the sample to a cantilever beam near the fulcrum; the small deflection of the beam is indicated by means of a differential transformer, which in turn is attached to an automatic recorder.The birefringence is measured with a Senarmont compensator (quarter‐wave plate method). Measurements can be made either manually or with a continuously rotating analyzer. This latter method permits the automatic recording of the birefringence and is also adaptable to the study of rapid birefringence changes. The sample is housed in a copper pipe surrounded by a thermostatically controlled liquid bath.Some data are presented showing the use of this apparatus in measuring birefringence and stress changes during stress relaxation experiments (sample held at constant elongation), birefringence changes during creep under constant load, and stress‐optical coefficients.