The effect of urea on the oviposition behaviour of culicine vectors of Japanese encephalitis was studied in rice fields. Gravid Mosquitoes use a complex integrated sensory system for selecting an oviposition site (Vrtiska & Pappas 1984).Long-range cues, probably involving vision, allow mosquitoes to identify different habitats (Bidlingmayer 1975) tritaeniorhynchus was not always adaptive in site selection, as the females were often found ovipositing on water unsuitable for larval development (Mogi 1978).The objective of the present study was to determine the role of urea, a synthetic nitrogenous fertilizer, as an oviposition attractant/stimulant for the mosquito vectors of Japanese encephalitis (JE) virus. During a routine survey of mosquito immatures, clustering of egg rafts was observed in a localized area in a rice field at Soundarasolapuram village, Cuddalore district, Tamil Nadu State. It was later found that the farmer had selectively treated this patch with urea on the previous day, as he felt the plants were stunted. This observation prompted this exploration of the role of urea on the oviposition behaviour of vectors of JE.This study is a part of the PhD Thesis submitted to Madurai Kamaraj University, Madurai, Tamil Nadu, India. A series of three experiments was conducted in rice fields 10 days after transplantation. Metal quadrats of size 0.1 m 2 open at both ends were used for the experiment. One end of the quadrat was pushed into the bottom mud of the rice field, while the other end remained at least 5 cm above the water surface, thus preventing the mixing of water in the quadrat and the main field. The water in the quadrat was filtered to remove insect predators. In the first experiment, three paddy fields were selected and four quadrats were placed equidistant from each other at least 25 m apart in each of the rice fields sampled in each field. Two quadrats (T1) were treated (surface-applied) with a dose of 1 g urea/0.1 m 2 , while the other two were kept as untreated controls (T2). In the second experiment, the dosage of urea was increased to 2 g urea/0.1 m 2 . In the third experiment, five transplanted rice fields were selected and three quadrats were placed in each field. One set of quadrat was treated with a dose of 1 g urea/0.1 m 2 (T1), the second set (T2) had a one l cylindrical plastic container of diameter 10 cm placed in it; which was filled with mud for 2 cm depth, and urea with similar dosage as that of T1 (1 g/0.1 m 2 ) was sprinkled in the mud filled container. The container was covered with mosquito netting, to prevent the gravid females from contacting the treated water surface, but allowed the volatile fractions to escape into the atmosphere. The container was placed in the centre of the quadrat with its mouth well above the water surface, and with enough space around it so that the gravid females had access to the water surface in the quadrat. The third set of quadrats (T3) without urea application was kept as control for comparison.In all the three experiments, the number of e...