Bivalves have been used to assess microplastics (MPs) from their surrounding waters and recently, bivalve active biomonitoring was proposed as a potential approach to quantify and assess MPs pollution in the field. However, key questions on the deployment duration and on the potential use of bivalves as a good proxy for MPs in the aquatic environment are still to be resolved. Here, we investigate the effect of exposure duration on caged blue mussels MPs bioaccumulation. The characteristics of ingested MPs were compared with those found in the surrounding sea surface water. Prior deployment, cultured mussels were depurated and transplanted in the marine coastal waters near an abandoned coastal landfill. Then, MPs bioaccumulation was studied 1, 2 and 5-weeks post transplantation. After 72-h depuration period, 98.8% of the MPs were eliminated. 1-week post deployment, the number of ingested MPs by caged mussels increased significantly to an average of 0.93 ± 0.6 items/g with the presence of fragments and/or fibers. During the second and the fifth weeks of deployment, the number of ingested MPs increased progressively, yet not significantly, and reached a threshold of 1.42 items/g. We found differences in the relative abundance of particle sizes and polymer types between seawater and mussels, with mussels over-representing particles