Microplastic (MP) primarily originate from degradation of plastic litter is a ubiquitous environmental contaminant. Its bioavailability and a so far insufficiently predictable environmental hazard potential require both, current and retrospective investigations. Blue mussels (Mytilus spp.) are an established indicator organism for monitoring environmental pollutants. As sediment filtering organisms they are in direct interaction with natural and anthropogenic deposition. Further they serve amongst others as a human food source. Therefore, they reflect environmental pollution and have a link to food safety and human health aspects. The bioavailability of MP for blue mussels has been shown and make them ideal indicators for retrospective studies. The German Environmental Specimen Bank is sampling and archiving blue mussels on a regular basis at Eckwarderhörne (North Sea) and at Darsser Ort (Baltic Sea). Samples of these sites are available from 1986 (Eckwarderhörne) and 1992 (Darsser Ort) up to now. Blue mussel subsamples of the available time series were pooled into duplicates. Samples were processed through an enzymatic and chemically oxidative digestion for MP extraction. Purified samples were analyzed mass-quantitatively for 9 polymers by pyrolysis gaschromatography-mass spectrometry (Py-GCMS) [1, 2]. Resulting data reflect temporal variations of 7 polymers in blues mussels at both locations. The most prominent polymers are PVC, PET and PS. This is independent from the respective site, although the samples from Eckwarderhörne show slightly higher contents for PVC and PET. The elevated abundance of high-density polymers like PET and PVC supposed to sink in marine waters might indicate their enrichment in the sediment and thus the blue mussel habitat. This study presents the first mass-quantitative MP data in biota over recent decades and allows a first insight into trends of microplastics contamination.  M. Fischer, B.M. Scholz-Böttcher, Environmental science & technology 51, 5052 (2017).  M. Fischer, B.M. Scholz-Böttcher, Anal. Methods 11, 2489 (2019).