Fire-induced changes in canopy openness may affect sunlight penetration to the forest floor, and thus the operative temperatures available to terrestrial ectotherms. We examined thermal regimes for two types of ectotherms: diurnally active species that utilize sun-exposed patches to regulate their body temperatures, and nocturnally active species that depend upon solar radiation striking the rocks under which they shelter. We measured canopy openness, shrub height, radiation transmission and operative environmental temperatures in the open and inside reptile retreat-sites, at 24 study sites in eucalypt forests in two regions (Gosford and Yengo) in south-eastern Australia. All sites were last burnt in 2000–2001, but had experienced different fire frequencies (1–4 fires over the previous 37 years). In Gosford, higher fire frequencies reduced canopy openness and radiation transmission at ground and shrub level, and thus reduced environmental temperatures and the thermal quality of reptile habitats. Our modelling based on thermal preferenda of an endangered snake species (the broad-headed snake Hoplocephalus bungaroides) suggests that increased fire frequency at Gosford halved the amount of time an animal could spend within its preferred (set-point) range, regardless of whether it thermoregulated beneath rocks or basked out in the open. At Yengo, however, fire frequency did not affect the thermal quality of reptile habitats. Thus, the effects of fire frequency on forest structure and the thermal environment at ground level differed between adjacent areas, and relatively small changes in canopy openness translated into major effects on thermoregulatory opportunities for reptiles. Although fire is a useful management tool for creating open habitats, we need to understand more about the effects of fire frequency on vegetation structure and thermal environment before we can use fire to manage habitats for reptiles.