ABSTRACT: Mediterranean mussels Mytilus galloprovincialis were experimentally cultured from 5 rafts located in 4 Galician Rías, following the established industrial procedure. Cultures were sampled monthly until mussels exceeded market size. Observation of histological sections of sampled mussels by light microscopy demonstrated symbionts which could be classified into 3 groups according to their pathogenicity. The first group consisted of symbionts with unnoticeable pathogenic effects including: prokaryotic inclusion bodies (PIB) in digestive gland and gills, an unidentified protistan in digestive primary ducts, a kidney coccidian, intracytoplasmic ciliates in digestive tubules, gill ciliates and a turbellarian in the intestinal lumen. The second group comprised symbionts that could damage the host, although unlikely to be lethal, including: the microsporidian Steinhausia mytilovum, the flatworm Urastoma cyprinae, and the copepod Mytilicola intestinalis. The third group included the protistan Marteilia refringens and the trematode Proctoeces maculatus, potentially lethal pathogens. In addition, mussels with haemocytic infiltration of tissues and granulocytomas and a few cases of disseminated neoplasia were detected. The qualitative composition of mussel symbiont community was similar at the 5 study sites, except for 3 symbionts which were not detected at some sites. Quantitatively, symbiont loads were higher and histological signs of stress more abundant in Moaña and Vilagarcía (the most inner sites in the Rías), intermediate in Illa de Arousa and Muros, and lower in Lorbé. Symbionts increased in prevalence as mussels grew. Some of the symbionts were detected in mussel seed at the beginning of the experimental cultures.