We investigated the role of invasive Pacific oysters (Magallana gigas) in determining parasite infection levels in native blue mussels (Mytilus edulis) in relation to other environmental and biotic factors. Type of research, method & collection of data: We used hierarchical field sampling covering three spatial scales along a large intertidal ecosystem (European Wadden Sea). We applied mixed models to analyze the associations between parasite prevalence and abundance in mussels and oysters, and 12 biological and environmental factors. Data were acquired by field observations, modelling exercises and literature studies.