A large number of sedimentary rock formation the Colorado Plateau and in the Wyoming foreland have been investigated paleomagnetically in an attempt to determine the geomagnetic polarity during the Middle and the early Late Jurassic, a period frequently postulated to have been of constant normal polarity. A survey of published paleomagnetic litature shows this postulate to be without grounds. A wide range of sedimentary lithologies, both red and nonred, were investigated. Most formations displayed a complex, multivectorial magnetization, even to very high stability ranges. However, a strikingly large amount of reversed polarity was observed and relatively little normal Jurassic polarity. Because of the widespread geographic data base, the multitude of lithologies, and the amount of time represented, it is tentatively concluded that the data at least partially reflect the character of the geomagnetic field during the Jurassic. Weak intensities in all formations, and the instability, may support the marine magnetic anomaly hypothesis of weak geomagnetic field intensity for this time. These data do not support the hypothesis of a long period of normal polarity during the Jurassic field might have frequencly been of reversed polarity.