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, tolerance of a broad range of habitats, 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 the ranges of all subspecies of traditionally defined C. viridis except viridis and nuntius (i.e., southern British Columbia in western Canada, through the western United States (east to the Rocky Mountains) to central Baja California in Mexico). 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). The species avoids arid areas such as the Mojave and Colorado Deserts. The elevational range extends from sea level to around 3,355 m asl (11,000 feet) but most localities are below 2,745 m asl (9,000 feet) (Basey 1976, Stebbins 2003, Campbell and Lamar 2004).

This species is represented by a large number (hundreds) of occurrences (subpopulations) (Campbell and Lamar 2004). The adult population size is unknown but presumably exceeds 100,000. This snake is fairly common in many areas, but is very rare in Baja California south of Sierra San Pedro Martir. The extent of occurrence, area of occupancy, and number of subpopulations are probably relatively stable; population size has declined in many areas in comparison to historical levels (e.g., see Ernst and Ernst 2003). Currently, extent of occurrence, area of occupancy, and number of subpopulations are probably relatively stable; population size may be declining but probably at a rate of less than 10% over 10 years or three generations.

This snake occupies a wide diversity of habitats, from shrubby coastal dunes to timberline, from shrubby basins, chaparral and canyons to open mountain forests (Nussbaum et al. 1983, Lowe et al. 1986, Brown et al. 1995, Hammerson 1999, Stebbins 2003, Campbell and Lamar 2004). It avoids very arid areas. 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 (Ashton and Patton 2001).

Overall, this species is not seriously threatened across most of its range. Locally, populations are being reduced as a result of persecution (especially around human settlements) and habitat loss and degradation from residential, commercial, and agricultural development. Populations in Baja California are being impacted by urbanization, agricultural expansion, and the construction of new roads (on which animals made be killed through collision with vehicles).

There are many occurrences in protected areas.