Plasmodium falciparum isolates were collected during a cross-sectional survey of fever cases in an endemic area of northern India, over a period of 6 months (July-December 1996). Smears of peripheral blood from 118 cases were found to contain Plasmodium falciparum. Isolates of the parasites were successfully cultured in vitro, under identical conditions, from blood samples from 58 of the smear-positives who had asexual parasitaemias of at least 0.05%. Gametocytes developed from 50 of the 58 isolates, with a variable range of gametocytaemia. An isolate was most likely to produce gametocytes and to produce high gametocytaemias (i.e. ≥ 1.0%) in vitro if it had been collected when the intensity of transmission (as reflected in bloodsmear positivity) was relatively high. A young child (< 6 years) with fever was more likely to be smear-positive for the asexual and sexual stages of P. falciparum, but less likely to be seropositive for antibodies to these stages, than the older cases of fever.