_Dipodomys spectabilis spectabilis_ Merriam requires comparison with three other forms of kangaroo rats in the same general region, namely, _D. deserti_ Stephens, of approximately the same size, and _D. merriami_ Mearns and _D. ordii_ Woodhouse, the last two of decidedly smaller size. The range of _deserti_ lies principally to the west of that of _spectabilis_, and the two do not, so far as known, overlap. On the other hand, _merriami_ and _ordii_, and subspecies, occur over a large part of the range of _spectabilis_, living in very close proximity to its burrows; _merriami_ is even suspected of pillaging the stores of _spectabilis_. The range of _merriami_, however, is much more extensive than that of _spectabilis_ (Fig. 1), which argues against a definite ecological dependence or relationship. Separation of the four forms mentioned may be easily accomplished by the following key: _Key to Species of_ Dipodomys _in Arizona._ _a^1_. Size much larger (hind foot and greatest length of skull more than 42 millimeters); tail tipped with white. _b^1_. Upper parts dark brownish buffy; tail dark brownish or blackish with more sharply contrasted white tip; interparietal broader, distinctly separating mastoids (range in Arizona mainly southeastern part) =Dipodomys spectabilis.= _b^2_. Upper parts light ochraceous-buffy; tail pale brownish with less sharply contrasted white tip; interparietal narrower, reduced to mere spicule between mastoids (range in Arizona mainly southwestern part) =Dipodomys deserti.= _a^2_. Size much smaller (hind foot and greatest length of skull less than 42 millimeters); tail not tipped with white. _b^1_. Hind foot with four toes =Dipodomys merriami.= _b^2_. Hind foot with five toes =Dipodomys ordii.= On account of the small size, _merriami_ and _ordii_ do not require detailed color comparison with the other two. The general color of the upperparts of _spectabilis_ is much darker than that of _deserti_; whereas _spectabilis_ is ochraceous-buff or light ochraceous-buff grizzled with blackish, _deserti_ is near pale ochraceous-buff and lacks the blackish. The color of the upperparts alone amply suffices to distinguish _spectabilis_ and _deserti_; but the different coloration of the tail is the most obvious diagnostic feature. The near black of the middle portion of the tail, the conspicuous white side stripes, and the pure white tip make the tail of _spectabilis_ stand in rather vivid contrast to the pale-brown and whitish tail of _deserti_.