Ten species of mosquitoes (Diptera: Culicidae) from five genera were exposed to preparasites of the tropical mermithid nematode species Romanomermis iyengari (Welch) (Nematoda: Mermithidae), a strain isolated in 1978 from Pondicherry. By exposing mosquito larvae during the second instar, nematode infection was invariably lethal, the rate being highest in Culex sitiens Wiedemann (95%) followed by Cx. quinquefasciatus Say (90%), Aedes aegypti (L.) (79%), Anopheles subpictus Grassi (64%), Ae. albopictus (Skuse) and Armigeres subalbatus Coquillett (62%), Cx. tritaeniorhynchus Giles (57%), Mansonia annulifera (Theobald) (46%), An. stephensi Liston (40%) and An. culicifacies Giles (36%). When fourth-instar larvae were exposed, the infection was highest in Ar. subalbatus (66%), followed by An. stephensi (52%), Cx. quinquefasciatus (47%), Ae. aegypti and An. subpictus (42%), Ae. albopictus (30%), An. culicifacies (29%), Cx. sitiens (24%), Cx. tritaeniorhynchus (19%) and Ma. annulifera (8%), with 2–45% of infected culicines surviving to adulthood. The parasitic phase of the nematode lasted 5–7 days in all the host species, yielding 1.1–3.2 parasites per II instar and 1.1–2.5 parasites per IV instar. The overall output of parasites per 100 mosquito larvae (infected + uninfected) was highest for Ae. aegypti when mosquitoes were exposed during II instar (2.53 parasites/larva) and for Ar. subalbatus when mosquitoes were exposed during IV instar (1.65/larva), and lowest for Ma. annulifera exposed during IV instar (0.09/larva). For routine laboratory culture of R. iyengari it is convenient to employ Cx. quinquefasciatus as the host yielding 90–190 parasites/100 larva.