This report describes the production of ERM?-CD200, a powdered bladderwrack material (Fucus vesiculosus) certified for the mass fraction of the total content of As, Cd, Cu, Hg, Pb, Se and Zn. The material was produced following ISO Guide 34:2009. Approximately 60 kg of brown algae seaweed (bladderwrack, Fucus vesiculosus) was collected in Galway (Ireland) and processed at IRMM (Belgium) to produce a certified reference material (CRM) of seaweed powder. The produced vials containing the processed seaweed were carefully capped, sealed and stored for further certification studies. Between-unit homogeneity was quantified as well as stability during dispatch and storage in accordance with ISO Guide 35:2006. Within-unit homogeneity was also quantified to determine the minimum sample intake. The material was characterised by an inter-laboratory comparison among laboratories of demonstrated competence and adhering to ISO/IEC 17025. Technically invalid results were removed but no outlier was eliminated on statistical grounds only. Uncertainties of the certified values were calculated in compliance with the Guide to the Expression of Uncertainty in Measurement (GUM) including uncertainty contribution related to possible heterogeneity and instability of the material as well as to the characterisation. The material is intended for the quality control and assessment of method performance. As any reference material, it can also be used for control charts or validation studies. The CRM is available in amber glass vials containing approximately 5 g of dried powder, placed in aluminized polythene sachet closed under a nitrogen atmosphere. The minimum amount of sample to be used is 200 mg. The CRM was accepted as European Reference Material (ERM?) after peer evaluation by the partners of the European Reference Materials consortium.