Abstract: What factors limit where a microbial species grows and thrives? The rate of clonal population growth is the primary measure used to assess microbial fitness in particular environments. However, net population growth is itself a function of two more fundamental demographic parameters – rates of birth and death. Analyzing changes in birth and death rates in response to environmental stress can help to identify physiological constraints that limit microbial niches; however measuring these demographic variables in microbial populations is challenging because it requires information about individual cells. In order to overcome this obstacle we developed a novel cellular “mark-release-recapture” technique that enables single cell measurement of age-structured birth and death rates in budding yeast. We uncovered a surprising variety of demographic explanations for reduced population growth during heat stress, including dramatic shifts in the age-structure of mortality resulting from either "early life mortality" or "premature senescence", depending on genetic background. Our findings suggest that a diverse set of physiological factors enforce the thermal niche of microbes.