The concentration and transport of faecal pellets (FPs) produced by blackfly (Diptera: Simuliidae) larvae were estimated in a large, free-flowing river in the north of Sweden during 1997–1999. FPs were abundant from May through August and FP loads in transport peaked at 429 t dry mass d−1 at a site in the lower part of the river in 1997. Daily transport at the same site, averaged over each study period (16 Jun.–18 Sep. 1997, 6 Apr.–10 Sep. 1998, and 21 Apr.–5 Aug. 1999), was estimated to be 93.7 t dry mass (3.7 t carbon), 47.5 t dry mass (1.9 t carbon and) and 69.2 t dry mass (2.7 t carbon), respectively. On a large scale, there was a downstream trend of increasing FP concentration and, during periods with greater discharge, sedimentation was reduced so that more material was exported from the river. Samples from six sites in a regulated river (into which our focal river flows) parallel to six sites in the unregulated tributary showed considerably lower FP concentrations in the regulated river, presumably because of much smaller blackfly populations as a consequence of habitat loss through damming. A survey of two other large, unregulated rivers in northern Sweden confirmed that these also carry large amounts of FPs. We conclude that the transformation of small suspended food particles into considerably larger FPs by huge populations of filter-feeding blackfly larvae is a major process in large northern rivers.