Samples of salmonids were taken from six Scottish rivers and examined for freshwater mussel (Margaritifera margaritifera (L.)) glochidiosis. The prevalence and magnitude of natural infections observed were comparable to those reported elsewhere. In most rivers, older fish seem to be less susceptible than 0+ fish, possibly due to an acquired immunity resulting from previous exposures. Initial infection loads may be greater on older fish due to greater ventilation rates and/or gill surface areas. However, this host size effect appears to be transitory. In a stock of farmed 0+ salmon, an entire parasitic stage was monitored. This took up to 11 months and only 5-10% of the initially attached glochidia managed to metamorphose and excyst as juvenile mussels. There are apparent differences in host utilisation between salmon and trout in certain rivers. Some trout stocks appear to be under-utilised by M. margaritifera, possibly due to differences in behaviour and/or spawning site. 0+ salmon are the most important hosts in several rivers. However, there are a number of mussel populations located in small streams which have no salmon, and these are entirely trout-dependent. This may be important in terms of conservation, with regard to the recent collapse of migratory trout stocks in Scotland.