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Williams et al. (1984) detail the life history of gillettii. The yellow eggs are laid in clusters on the host plant, and larvae feed communally in loose silken nests in the early instars. They are able to overwinter in second, third or fourth instars, and take two years to develop in some localities. Mature larvae are dark brown and black with a yellow dorsal stripe and white lateral band. This checkerspot is dependent on moist clearings in montane or lower subalpine conifer forest, often found along watercourses. The Yellowstone fires of 1988 created suitable new habitat for this species (Williams 1995). It appears to be declining in other parts of its range, at least in part due to forest fire suppression (Debinski 1994).

This species is endemic to the Rocky Mountains, and has one of the most geographically restricted ranges of any North American butterfly. Occurs from Nordegg, AB south to Wyoming and Idaho (Bird et al. 1995, Scott 1986). The Nordegg record, stemming from 1918, has not been repeated in recent times, and the nearest known populations occur in the Bow Valley, 150km to the south.