The global annual production of plastics increased 20-fold in the last five decades, reaching about 335 million tons in 2016. Concomitantly, environmental pollution by plastic litter became a rising issue. Plastics easily escape from landfills into the surrounding environment and rivers, which discharge huge quantities of litter into the oceans. Plastic debris spoil the coastlines or accumulate in subtropical oceanic gyres. Marine plastic litter can have adverse effects on marine vertebrates and invertebrates. Larger items, such as lost fishing nets are particularly hazardous because many marine species may become entangled and immobilized. Limited predator avoidance, starvation or drowning may be the consequences. Plastics are hardly biodegradable but subject to mechanical degradation by wave action and UV-radiation. Progressive fragmentation of larger plastic items generates smaller fragments, finally yielding micro- or even nanoparticles. Upon ingestion, smallest plastic fragments may enter organs and even penetrate into cells where they may cause imbalances of the cells homeostasis. In the present study, the ingestion of microplastics by marine invertebrates, the possible transfer into cells of the digestive tract, and the cellular effects in the midgut gland were studied. The Atlantic ditch shrimp (Palaemon varians) served as model species. It inhabits coastal regions, estuaries, and brackish water systems, which are strongest exposed to anthropogenic pollution. The shrimps received fluorescent polystyrene microbeads of 0.1, 2.1, and 9.9 μm in diameter as food. Uptake of the microbeads into the digestive organs and, particularly, into the midgut gland was analysed by fluorescent microscopy of cryostat sections of the digestive tract. Activities of antioxidant enzymes were measured to verify cellular stress responses. Amplification of NADPH-oxidase transcripts served as indicator for the presence of a cellular defence system capable of generating reactive oxygen species. The smallest particles penetrated into the midgut gland while the largest particles retained in the stomach. An increase in the cellular defence mechanism against oxidative stress was verified by the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT). Formation of reactive oxygen species (ROS) indicated NADPH oxidase activity, a superoxide (O2-) generating enzyme. The expression of NADPH oxidase in the midgut gland of P. varians was verified by PCR-amplification. The midgut gland of P. varians showed an intense cellular reaction after exposure to microplastics. The rapid increase of the anti-oxidative enzymes, particularly SOD, indicates a significant liberation of reactive oxygen species, presumably as an immune reaction of the NADPH-oxidase system. Oxidative stress, in turn, can have adverse effects on various cell structures and cell functions by affecting membranes, proteins, or DNA. Finally, it causes a toxicological impact on organs and the whole organisms. Future studies should address the direct effects of increased oxidative stress in terms of toxicity, i.e. by the occurrence of lipid peroxidation. Also the destination of particles inside the organisms, may help identify corresponding mechanisms that lead to oxidative stress and contribute to the big picture of microplastics in the environment.