Ricefields constitute the most important breeding places of Anopheles aconitus which known as malaria vector in Java and Bali. With the increase in environmental awareness, the use of biological agents for pest control appears to have been revived especially for larva. The larval parasites such as mermethid nematodes Romanomermis iyengari have been shown effective to use in open field mosquito habitats like ricefields. However the use of pesticides for controlling agricultural pests at least have an impact to nontarget organism who live in the waters. The preparasite stage of R. iyengari could not invade the second stage instar of An. aconitus larva if the media contains of Confidor 5 WP with the dosages of 0.0038 mg/l; 0.00958 mg/l and 0.01816 mg/l.