Ionizing radiation is known to effect development during early life stages. Lake whitefish (Coregonus clupeaformis) represent a unique model organism for examining such effects. The purpose of this study was to examine how ionizing radiation affects development in lake whitefish embryos and to investigate the presence of an adaptive response induced by heat shock. Acute exposure to ¹³⁷Cs gamma rays was administered at five time points corresponding to major developmental stages, with doses ranging from 0.008 to 15.5 Gy. Chronic gamma-ray exposures were delivered throughout embryogenesis within a custom-built irradiator at dose rates between 0.06 and 4.4 mGy/day. Additionally, embryos were given a heat shock of 3, 6 or 9°C prior to a single acute exposure. Radiation effects were assessed based on survival, development rate, morphometric measurements and growth efficiency. Embryos showed high resistance to acute exposures with an LD_50/hatch of 5.0 ± 0.7 Gy immediately after fertilization, increasing to 14.2 ± 0.1 Gy later in development. Chronic irradiation at all dose rates stimulated growth, with treated embryos up to 60% larger in body mass during development compared to unirradiated controls. Chronic irradiation also accelerated the time-to-hatch. A heat shock administered 6 h prior to irradiation reduced mortality by up to 25%. Overall, low-dose chronic irradiation caused growth stimulation in developing lake whitefish embryos and acute radiation mortality was reduced by a heat-shock-induced adaptive response.