Epibionts of mussels can have detrimental effects on their basibionts, such as reduced growth rates, lower fecundity, increased mortality and an enhanced risk of dislodgement of the overgrown bivalves due to stronger hydrodynamic forces. In blue mussels Mytilus edulis, the epibiotic American slipper limpet Crepidula fornicata reduces growth and survival. In a field experiment we tested the hypothesis that an enhanced byssus thread production with high energetic costs for the mussels due slipper limpet epibionts is the underlying mechanism for the epibiont burden. Byssus thread production in overgrown mussels was twice as high as in unfouled M. edulis (11 ± 0.9 and 5.4 ± 0.6 byssus threads/mussel/day, respectively). A control experiment revealed intermediate byssus thread production (8.4 ± 0.8 byssus threads/mussel/day) in mussels cleaned of C. fornicata at the beginning of the experiments, indicating that C. fornicata is responsible for the effects observed. We conclude that increased byssus production in fouled M. edulis is a functional response to higher drag caused by epibionts and that it is associated with increased energy expenditure that reduces allocation of resources for other processes such as growth, reproduction and survival. Such indirect effects of epibionts, mediated by an enhanced byssus production, may be widespread in byssus-producing bivalves, with important implications for their population dynamics. © The Author 2007. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved.