Abstract. The limited available evidence about effects on marine fishes of high CO₂ and associated acidification of oceans suggests that effects will differ across species, be subtle, and may interact with other stressors. This report is on the responses of an array of early life history features of summer flounder (Paralichthys dentatus), an ecologically and economically important flatfish of the inshore and nearshore waters of the Mid-Atlantic Bight (USA), to experimental manipulation of CO₂ levels. Relative survival of summer flounder embryos in local ambient conditions (775 μatm pCO₂, 7.8 pH) was reduced to 48% when maintained at intermediate experimental conditions (1808 μatm pCO₂, 7.5 pH), and to 16% when maintained at the most elevated CO₂ treatment (4714 ppm pCO₂, 7.1 pH). This pattern of reduced survival of embryos at high-CO₂ levels at constant temperature was consistent among offspring of three females used as experimental subjects. No reduction in survival with CO₂ was observed for larvae during the first four weeks of larval life (experiment ended at 28 d post-hatching (dph) when larvae were initiating metamorphosis). Estimates of sizes, shapes, and developmental status of larvae based on images of live larvae showed larvae were initially longer and faster growing when reared at intermediate- and high-CO₂ levels. This pattern of longer larvae – but with less energy reserves at hatching – was expressed through the first half of the larval period (14 dph). Larvae from the highest-CO₂ conditions initiated metamorphosis at earlier ages and smaller sizes than those from intermediate- and ambient-CO₂ conditions. Tissue damage was evident in larvae as early as 7 dph from both elevated-CO₂ levels. Damage included dilation of liver sinusoids and veins, focal hyperplasia on the epithelium, and separation of the trunk muscle bundles. Cranio-facial features changed with CO₂ levels in an age-dependent manner. Skeletal elements of larvae from ambient-CO₂ environments were comparable or smaller than those from elevated-CO₂ environments when younger (7 and 14 dph) but were larger at developmental stage at older ages (21 to 28 dph), a result consistent with the accelerated size-development trajectory of larvae at higher-CO₂ environments based on analysis of external features. The degree of alterations in the survival, growth, and development of early life stages of summer flounder due to elevated-CO₂ levels suggests that this species will be increasingly challenged by future ocean acidification. Further experimental studies on marine fishes and comparative analyses among those studies are warranted in order to identify the species, life stages, ecologies, and responses likely to be most sensitive to increased levels of CO₂ and acidity in future ocean waters. A strategy is proposed for achieving these goals.