Ambystoma gracile adults may be either metamorphosed terrestrial salamanders or neotenic water-dwellers.

Metamorphosed adults measure 14-22 cm in total length (Boundy and Balgooyen 1988). They are dark brown, gray, or black above and a lighter brown below. A glandular ridge forms a rounded top edge on the tail; the lower edge of the tail is sharp (Petranka 1998; Snyder 1963). Conspicuous oval parotoid glands sit immediately behind the eyes. The tail ridge appears rough due to concentrations of granular glands (Brodie and Gibson 1969). Both the parotoid and tail ridge glandular areas are pitted with openings of poison glands. Broad head, relatively small eyes. No tubercles on the underside of the feet (Stebbins 1985). Sometimes terrestrial individuals have small, irregular, whitish or yellowish spots and blotches on their dorsum (see Comments section below).

Neotenic adults are strictly aquatic and retain gills. They may grow as large as 13 cm in snout-vent length and 26 cm in total length (Boundy and Balgooyen 1988). They are brownish to olive green in color, mottled with yellow and black. Distinct yellow spots sometimes sometimes dot the sides and tail. Ventral color varies from cream to pale or dark gray (Petranka 1998). The parotoid glands of untransformed adults are less prominent than those of transformed adults (Licht and Sever 1993).

Both terrestrial and neotenic males become darker than females during the breeding season. Unmetamorphosed adult males have hypertrophied hindlimbs and feet, are less spotted, and have an enlarged glandular ridge on the tail (Snyder 1956).

Larvae belong to the pond type, having long gills, long toes, and a long dorsal fin (Stebbins 1985). Hatchlings average about 8 mm in snout-vent length (Licht 1975). They have concentrated dark pigment along the base of the dorsal fin. Older larvae vary in color from dark brown to olive green or light yellow. They often have sooty blotches on the dorsum and may have yellow flecks or spots along the sides (Stebbins 1985).
 
No morphological differences have been found to distinguish neotenic larvae from those that will transform (Snyder 1963).

The spotted and unspotted varieties were originally designated as separate subspecies, with Ambystoma gracile decorticatum (British Columbia Salamander) possessing spots and Ambystoma gracile gracile (Brown Salamander) lacking spots. The spotted form occurs in the northern part of the range for A. gracile. A dividing line of 51 degrees north latitude was recognized between the two subspecies, with A. g. decorticatum occurring north of the line and A. g. gracile living to the south (Snyder 1963). Morphological characters such as the presence of 3 instead of 4 phalanges on the fourth toe, more prominent parotoid glands, and rows of 4 rather than 2 teeth on the prevomers have been used to differentiate between the unspotted A. g. gracile and the spotted A. g. decorticatum (Dunn 1944). However, Titus (1991) found that these distinctions do not always hold true and that genetic variation does not always correlate well with subspecies, and he suggested that A. gracile should not be split into subspecies.

The specific epithet gracile is derived from the Latin term gracilis which means "slender" or "delicate." The subspecific name decorticatum is taken from the Latin de ("from") and cortex ("bark" or "cork"), meaning "like bark" (Snyder 1963).

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