There is global concern regarding the fate and effects of microplastics in the environment, particularly in aquatic systems. In this study, ethylene acrylic acid copolymer particles were evaluated in a chronic toxicity study with the aquatic invertebrate, Daphnia magna. The study design included a natural particle control treatment (i.e., silica) in order to discern any potential physical effects of a particlefrom intrinsic toxicity of the test material. In addition to the standard endpoints of survival, growth, and reproduction, the transcriptomic profile of control and ethylene acrylic acid copolymer-exposed D. magna were evaluated at the termination of the 21-day toxicity study. No significant effects on D. magna growth, survival, or reproduction were observed in the study in comparison to both particle and untreated control groups. Significant transcriptomic alterations were induced in the highest treatment level of 2.3 x 1012 particles of the ethylene acrylic acid copolymer/ L in key pathways linked to central metabolism and energy reserves, oxidative stress, as well as ovulation and molting indicating a global transcriptomic response pattern. To put the results in perspective is challenging at this time, since, to date, microplastic environmental monitoring approaches have not been equipped to detect particles in the nano size range. However, the results of this study indicate that ethylene acrylic acid copolymer microplastics in the upper nano-size range are not expected to adversely affect D. magna growth, survival, or reproductive outcomes at concentrations up to 1012 particles/L. Overall design: There are 5 samples exposed to vehicle control, and 5 samples each are exposed to one of either 2.3x10^11 particles/L of ethylene acrylic acid copolymer (nanolow), 2.3x10^12 particles/L of ethylene acrylic acid copolymer (nanohigh) or 2.3x10^12 fumed silica.