The rate of emergence of micropredatory gnathiid isopods from the benthos, the proportion of emerging gnathiids potentially eaten by Labroides dimidiatus, and the volume of blood that gnathiids potentially remove from fishes (using gnathiid gut volume) were determined. The abundance (mean ±s.e.) of emerging gnathiids was 41·7 ± 6·9 m−2 day−1 and 4552 ± 2632 reef−1 day−1 (reefs 91–125 m2). The abundance of emerging gnathiids per fish on the reef was 4·9 ± 0·8 day−1; but excluding the rarely infested pomacentrid fishes, it was 20·9 ± 3·8 day−1. The abundance of emerging gnathiids per patch reef was 66 ± 17% of the number of gnathiids that all adult L. dimidiatus per reef eat daily while engaged in cleaning behaviour. If all infesting gnathiids subsequently fed on fish blood, their total gut volume per reef area would be 17·4 ± 5·6 mm3 m−2 day−1; and per fish on the reefs, it would be 2·3 ± 0·5 mm−3 fish−1 day−1 and 10·3 ± 3·1 mm3 fish−1 day−1 (excluding pomacentrids). The total gut volume of gnathiids infesting caged (137 mm standard length, LS) and removed from wild (100–150 mm LS) Hemigymnus melapterus by L. dimidiatus was 26·4 ± 24·6 mm3 day−1 and 53·0 ± 9·6 mm3 day−1, respectively. Using H. melapterus (137 mm LS, 83 g) as a model, gnathiids had the potential to remove, 0·07, 0·32, 0·82 and 1·63% of the total blood volume per day of each fish, excluding pomacentrids, caged H. melapterus and wild H. melapterus, respectively. In contrast, emerging gnathiids had the potential of removing 155% of the total blood volume of Acanthochromis polyacanthus (10·7 mm LS, 0·038 g) juveniles. That L. dimidiatus eat more gnathiids per reef daily than were sampled with emergence traps suggests that cleaner fishes are an important source of mortality for gnathiids. Although the proportion of the total blood volume of fishes potentially removed by blood-feeding gnathiids on a daily basis appeared to be low for fishes weighing 83 g, the cumulative effects of repeated infections on the health of such fish remains unknown; attacks on small juvenile fishes, may result in possibly lethal levels of blood loss.