Several investigators in the past one or two years have reported rheological data on systems capable of formation of crystalline phases during flow. Berens and Folt, and Collins and Krier reported data on polyvinyl chlorine systems which could be interpreted as a phase transition during melt flow. Kobayashi has reported on stress induced crystallization of polyethylene melts and Vinogradov has presented data on the formation of super‐molecular structures in isotactic polystyrene melts during flow. This paper is a report of our initial findings on a rheological transition in the isotactic polypropylene system.