Biofouling of external tags can damage both the animal and the tag. It is an inherent problem to many fish-tagging studies; however, it has received little attention. Between 2005 and 2007, we investigated the taxonomic group and wet weight of sessile multicellular eukaryonts growing on 33 B-type dart tags recaptured from raggedtooth sharks (Carcharias taurus Rafinesque, 1810) along the eastern coast of South Africa. All tags, except one from a shark that had been at liberty only for 13 days, contained some degree of macro-fouling that ranged from 0.031 to 7.687 g wet weight. Barnacles and hydroids were the first colonisers and were recorded on tags after only 47 days. Bivalves, polychaetes and algae succeeded within 60 days. The last two taxa to be recorded were ascidians and sponges. There was no significant relationship between either the biomass of sessile organisms (P = 0.17) or the number of taxa (P = 0.59) found growing on tags and their time at liberty. Calcium carbonate plugs, ranging in weight from 0.16 to 9.18 g, were found at the site of the tag insertion wound on 27.3% of recaptures. The present study highlights the extent of biofouling growth on tags and its potential to bias results from tagging studies.