These datasets comprise data obtained during a 42 week-long exposure experiment of Mytilus spp. to two types of microplastics, namely spherical polystyrene (PS) particles and irregular polyvinylchloride (PVC) particles. This data was obtained from January 2018 to January 2019 from juvenile blue mussels with an initial size of 10mm. We recorded body condition index. Body condition was measured three times, at week 0, week 16 and week 32. At week 0 only one group of mussels not exposed to microplastic particles was measured, at the other two time points mussels were exposed to three concentrations of PS (15, 1500, 15 000 particles/individual/week) and five concentrations of PVC (15, 1500, 15 000, 150 000 and 1 500 000 particles/individual/week) and one group to no particles at all as control.