Larvae, pupae, and adult blackflies (Simuliidae) are capable of harbouring several groups of nematodes. Multiple parasitism involving nematodes of the same or different species, or various developmental stages of the same nematode may also occur. Thus, blackflies can serve as paratenic, intermediate, or definitive hosts of nematodes. The present study reviews the literature pertaining to the association between nematodes and Simuliidae and provides a key to various groups of nematodes found in blackflies. Emphasis is placed on the commoner filarial and mermithid nematodes, but several other unusual and rare roundworm parasites are also discussed.