The first occurrences of wild oat resistance to diclofop in the Willamette Valley of Oregon were reported in 1990. Among eight resistant biotypes, GR50 values for diclofop were 3 to 64 times greater than the GR50 for a susceptible wild oat biotype. GR50 values for other aryloxyphenoxypropionate herbicides varied from 1 to over 100 times greater than a susceptible biotype. Only one resistant biotype was resistant to cyclohexanedione herbicides, and this was only a three-fold increase in GR50. Except for one biotype that had a low level of resistance to pronamide, none of the wild oat biotypes were cross-resistant to any other commonly used wild oat herbicide. Levels of resistance and cross-resistance did not follow a consistent pattern among biotypes in this study, suggesting more than one resistance trait. There were significant differences in the light use efficiency, height, dry weight, leaf area, and extent and timing of tillering and flowering of four wild oat biotypes studied. These physiological and morphological differences suggest that these resistant biotypes were selected independently. The diversity of resistance patterns and the coevolution of resistance at several locations will add to the difficulty of resistance management