We evaluated the thermal ecology of Crotalus lutosus in a semi-arid montane landscape of eastern Nevada (Great Basin National Park). During 2023–2024, we surgically implanted 29 snakes with temperature dataloggers to record field active body temperature (Tb) and measured environmental temperatures in a wide range of available microhabitats. We determined the species’s preferred body-temperature range (Tset = 28–32°C) from previous studies and calculated thermoregulation indices to evaluate thermoregulatory effectiveness (E). We hypothesize that gravid females exhibit higher median Tb and greater E than non-gravid females and males, with more frequent exceedances of the upper bound (32°C) in midsummer. We expected the effectiveness of thermoregulation to peak in the early morning and late afternoon of the day, and then decline at dusk and night, reflecting avoidance of midday thermal stress and cooler nighttime conditions. We assumed snakes shift among microhabitats—basking in sun-exposed sites when the environment is cooler than their preferred body temperature range and retreating under rocks when the surrounding area is too warm. We further predicted that E will decline over time across the active season as ambient temperatures cool. This study linked microhabitat choice, thermal landscapes, and thermoregulatory metrics to clarify how C. lutosus maintains body temperatures within preferred limits across time.