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blood-red over the lower sides and ventral region, especially in mature males. Although a genetic basis exists to express characteristic color patterns, the actual manifestation of 'color intensity and pattern depends upon age, sex, and diet" (see Figure 1). A summary of meristic characteristics for various Colorado subspecies of O. clarki (=5almo clarki) are provided in Figure 3. ' Although there is a close relationship between greenbacks and Colorado River cutthroat trout, recent mitochondrial DNA studies indicate that both the Arkansas River and South Platte River greenbacks are more closely related to each other, than to populations of Colorado River cutthroat. Greenbacks from the Arkansas and South Platte River drainages are nearly identical in DNA fragment patterns (Proebstel 1993). However, because of the geographic separation of the drainages, greenbacks from the two drainages should not be mixed for restoration purposes. Since greenback cutthroat trout hybridize with other species and subspecies of Oncorhvnchus, populations can range phenotypically from "essentially pure" to obvious hybrids. The Colorado Division of Wildlife (CDOW) has adopted a rating system developed by Binns (1977) as a means of rating population purity. Each population is assigned a letter ranging from A (pure) to C (obvious hybrids). ' Only Type A populations are considered for recovery purposes in this plan (Tables 1-4). However, known type B and C greenback populations (Table 5) are also included in hopes that information obtained from research on types A through C populations will be of value in formulating management plans for all ' cutthroat trout subspecies. Current Distribution ' The greenback cutthroat trout currently occurs in 51 sites that total 100 hectares of lakes and 146 kilometers of stream habitat in the upper tributaries of the South Platte and Arkansas river drainages. Nine "historic" populations remain that have been identified through recovery efforts conducted since 1973. Pure greenbacks have been introduced into 42 additional streams and lakes within the species historic range (as described in Objective 2, Part II of the Recovery section). u 5