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An investigation of the life history of the western chuckwalla, Sauromalus obesus, was undertaken in the Colorado Desert of southeastern California during a 7-year period. The study provided for a test of current life history theory and the modeling of population dynamics from environmental parameters. Colorado Desert S. obesus were mainly active February through September, but feeding occurred throughout the year. Coyotes were this species' primary predator. Home range sizes of males and females were correlated with adult female activity and nutrient requirements, respectively. The breeding status of females was determined by the presence of preovulatory follicles, oviductal eggs and copora lutea. The breeding status of males was best determined by the presence of sperm in the vas deferens and by levels of spermatogenic activity. Adult females always oviposited during the first half of July. Size at reproductive maturity for both sexes was about 125 mm snoutvent length. Age at maturity for males and females was 2 and 3 years, respectively. Mean annual reproductive frequency was 51%. Reproduction occurred in 6 of 7 years. Mean clutch size was 6.9 eggs. Clutch masses and egg masses averaged 34.3% and 5.3% of total body mass, respectively. For a given body size, there was no annual variation in clutch size, egg mass, or reproductive effort. The consistency of these traits indicates adaptation to a predictable environment. Relatively high egg masses are an adaptation to counter the harsh environment. Annual recruitment was about 20%, almost exclusively due to reproduction. First year and subsequent annual survivorship rates averaged 40% and 75%, respectively. Most individuals lived no longer than 10 years. Compared with Mojave Desert populations, Colorado Desert S. obesus demonstrated earlier maturity, higher reproductive rates, higher first year survival and lower adult survivorship. High reproductive rates and first year survival were attributable to the predictability of mild winters and summer rainfall. Such conditions promoted lower adult survivorship because of associated costs of reproduction and predation. Attempts were made to predict population age class structure from winter precipitation regression models. Such a procedure appears reliable for predicting age-specific fecundity and therefore is a useful tool for management practices.