Analysis of historical biogeography based on mtDNA data (Pook et al. 2000) revealed two main clades, one including populations from east and south of the Rocky Mountains and the other consisting of populations west of the Rocky Mountains. The conventionally recognized subspecies do not fully correspond to the phylogenetic pattern, and a review of the systematic status of several populations is needed (Pook et al. 2000).

Ashton and de Queiroz (2001) examined mtDNA variation among 26 populations of C. viridis and also identified two main clades: eastern, including subspecies viridis and nuntius (low levels of genetic divergence), and western, including all other subspecies. However, Ashton and de Queiroz (2001) differed from Pook et al. (2000) with respect to the relationships among members of the western clade, although Ashton and de Queiroz studied only a few individuals from each member of the western clade and stated that the relationships within the western clade are largely unresolved and that none (except possibly cerberus) appeared to deserve recognition as separate evolutionary species. Ashton and de Queiroz suggested that the two main clades be regarded as distinct species, C. viridis (eastern clade) and C. oreganus (western clade). The historical biogeographic scenario described by Ashton and de Queiroz (2001) suggests secondary contact between C. viridis and C. oreganus in northern Arizona, southwestern and northwestern Colorado, and southeastern Utah.

Douglas et al. (2002) examined mtDNA variation in C. viridis, with emphasis on the populations on the Colorado Plateau. As did Pook et al. (2000) and Ashton and de Queiroz (2001), they identified eastern and western clades, with the former including the nominal subspecies viridis and nuntius and the latter encompassing all of the other subspecies. Douglas et al. (2002) argued that all of the western subspecies should be recognized as species, but they did not effectively indicate details of distributional relationships in the contact zones among the proposed species. Douglas et al. (2002) concluded that the taxon nuntius should be regarded as a synonym of viridis.

Crother et al. (2003) considered all of the foregoing evidence and adopted the two-species taxonomy (Crotalus oreganus, Crotalus viridis) that is supported by the congruence among all three studies cited above. Campbell and Lamar (2004) also recognized only the two species. However, further clarification of the distributions of C. viridis and C. oreganus is needed, particularly in the contact zones in northern Arizona, southwestern and northwestern Colorado, and southeastern Utah. For example, populations in northwestern Colorado (Moffat County) identified by Douglas et al. (2002) as C. viridis were mapped as C. oreganus concolor by Campbell and Lamar (2004). We follow Crother et al. (2003) and Campbell and Lamar (2004) in recognizing C. oreganus and C. viridis as separate species.

Listed as Least Concern in view of its wide distribution, presumed large population, and because it is unlikely to be declining fast enough to qualify for listing in a more threatened category.

As defined by Crother et al. (2003), following congruence of Pook et al. (2000), Ashton and de Queiroz (2001), and Douglas et al. (2002), this species encompasses only the ranges of subspecies viridis and nuntius of traditionally defined C. viridis. In other words, the range extends from southern Alberta and southern Saskatchewan in Canada, to the northern fringe of northern central Mexico, west to Idaho, Wyoming, Colorado, and extreme eastern Arizona, east to the Dakotas, western Iowa, Nebraska, central Kansas, central Oklahoma, and western and central Texas in the United States (Stebbins 2003, Campbell and Lamar 2004). The ranges and relationships of Crotalus oreganus and Crotalus viridis in the Four Corners region and in northwestern Colorado need further clarification (Hammerson 1999; Brennan and Holycross, 2004). Its elevational range extends from about 100 m asl near the Rio Grande (Campbell and Lamar 2004) to at least 2,895 m asl (9,500 feet) in Colorado (Hammerson 1999).

This species is represented by a very large number of occurrences. On a range-wide scale, Campbell and Lamar (2004) mapped hundreds of collection sites (see also dot maps in Degenhardt et al. 1996 and Hammerson 1999). The adult population size is unknown but certainly exceeds 100,000. Some local populations have declined or disappeared as a result of historical killing of snakes at dens (Hammerson 1999, Ernst and Ernst 2003). Currently, its extent of occurrence, area of occupancy, and number of subpopulations, and population size are probably relatively stable or declining at a rate of less than 10% over 10 years or three generations.

This snake inhabits a wide diversity of habitats, from prairies and arid basins to wooded mountains (Lowe et al. 1986, Degenhardt et al. 1996, Hammerson 1999, Werler and Dixon 2000, Stebbins 2003, Campbell and Lamar 2004, Werner et al. 2004). It is primarily terrestrial but sometimes climbs into trees or shrubs. When inactive, it occupies mammal burrows, crevices, caves, or similar secluded sites. Pregnant females may congregate near the winter den until parturition (Gannon and Secoy 1985, Graves and Duvall 1992).

No major threats are known. Locally, populations have been eliminated or depleted as a result of killing at dens and loss/degradation of habitat by residential, commercial, and agricultural development (Hammerson 1999).

Many occurrences of this species are in protected areas.