In order to conserve the freshwater pearl mussel in Ireland, populations that have a high risk of extinction must be identified and given priority for conservation. Growth of freshwater pearl mussels has been found to vary among populations on a wide geographic scale as well as on a local scale. Populations having a high growth constant (k), because of the small size of individuals and their shorter life-span and thus lower reproductive output, may be more likely to become extinct than those which have a low k and hence larger size and greater reproductive output. This study attempts to estimate the growth constant (k) in rivers in Donegal and Northern Ireland based on measuring the largest shell in each population. Large differences in values of k were found among rivers and these are discussed in relation to catchment bedrock types and the identification of conservation priorities. Approprlate conservation strategies are recommended for Margaritifera margaritifera populations in the north of Ireland.