Stressful experiences in early life can have profound and lasting impacts on phenotypic development. In anthropogenic environments, organisms are increasingly exposed to evolutionarily novel stressors that may play a major role in shaping the phenotypic variation upon which natural selection acts. For instance, ionizing radiation persists in areas affected by nuclear reactor accidents, nuclear weapons testing, and the nuclear power production process. This thesis explored the dose-response effects of early life ionizing radiation exposure on life- history traits in the cricket (Acheta domesticus L.). Specifically, this work had two goals: (1) to examine the effects of early life radiation stress on the potential tradeoff between growth and self-maintenance, and (2) to explore the dose-dependent effects of juvenile radiation exposure on adult trait expression (particularly fecundity, offspring investment, and offspring fitness). It was found that exposure to moderate doses of radiation in early development resulted in a slower juvenile growth rate but increased survival in early adulthood, suggesting that self- maintenance was prioritized over growth. Despite the strong inhibitory effects of early life radiation on adult female body mass, age-specific fecundity was negatively impacted only at relatively high radiation doses. Crickets exposed to moderate radiation doses in early development laid larger eggs in adulthood relative to controls and these eggs had a greater hatching success, suggesting that radiation exposure had transgenerational effects on offspring performance. No noticeable effects of early life radiation exposure were detected on total and non-enzymatic antioxidant capacity or hydrogen peroxide levels in adult females. Together, this research indicates that a single, acute exposure to ionizing radiation in early life can affect phenotypic development in a complex, dose-dependent manner and that rather than being purely negative, phenotypic responses can be sustained or even enhanced. / Thesis / Master of Science (MSc) / Stressful experiences in early life can have lasting impacts on organismal development, but the extent to which early life stress is harmful is unclear. In human-dominated environments, pollutants such as ionizing radiation can be a significant form of developmental stress. This thesis explores the fitness-related effects of early life ionizing radiation exposure in the cricket, Acheta domesticus. Impacts on growth, longevity, fecundity, and offspring fitness are explored. This research indicates that early life stress can affect phenotypic development in a complex, dose-dependent manner and that rather than being purely negative, responses can be sustained or even enhanced.