In areas of sympatry with Myotis yumanensis yumanensis, Myotis lucifugus is identifiable by darker ears, more gently sloping forehead, burnished hair tips, and greater mastoid breadth. Of 354 specimens tested by multivariate discriminate analysis, three are intermediate and judged probable hybrids. The area of California draining to Owens Lake supports a distinctive population of bat, herein given subspecific status. All Yuma myotis from the area of concern are considered M. y. yumanensis; this bat varies in size clinally from small in the southwest (Sonora) to large in northeastern Utah-northwestern Colorado. Direct breeding continuity between the Lower and Upper Colorado River populations is interrupted by the Mogollon Rim and associated highlands; thus gene flow between these areas is by way of the Rio Grande to the east.