Background/Question/Methods Microplastic (MP) pollution is a problem of growing concern, but relatively little is known about the effects of MPs on aquatic life. MPs (defined as particles < 5 mm in diameter) may become introduced to aquatic environments through discharge of wastewater effluent containing MP particles from manufactured products such as cosmetics and cleaning products (primary MPs), or in situ as from the breakdown of larger plastic debris into smaller particles (secondary MPs). Many MPs are within the size range of seston consumed by suspension feeders such as mussels and herbivorous zooplankton. So, those types of animals may be especially vulnerable to negative effects of MP pollution since MPs that are ingested may have toxic effects, and may likely interfere with the ability of the animals to obtain nutrition. In this 21-day laboratory study, we examined the effects of different concentrations of microplastic beads (4 ±1.5 μm in diameter) on the growth and reproduction of six clones of the zooplankter Daphnia pulicaria. Daphnia were chosen as the study species because they are suspension feeders likely to consume MP particles, are important components of the food webs of lentic freshwater ecosystems, and because they are a model organism used in toxicological studies. Results/Conclusions Overall, results from this experiment showed that exposure to MPs had the biggest impact on aspects of Daphnia reproduction (offspring produced per day, day of first reproduction). Daphnia exposed to intermediate (104 L-1) and high (105 L-1) levels of MPs produced significantly fewer offspring per day (p < 0.001, Kruskal-Wallis rank sum test) compared to the low MP treatment (103 L-1) and the control treatment. Individuals in the highest MP level treatment had a significantly delayed time to first reproduction when compared to Daphnia exposed to the other MP levels or the control (p = 0.04 for MP treatment effect, two-way ANOVA). Levels of survival and somatic growth were also lower for the MP treatments compared to the control, but differences for those variables were not statistically significant. For the most part, the growth, survival and reproduction responses were similar among the six clone types assayed in this experiment (exception: one clone had a significantly higher average growth rate than the others). So, the findings of this study indicate that MP concentrations in the range of 104-105 L-1 impairs reproduction in Daphnia, and that these negative effects on reproduction were consistent across the six clones assayed in this study.