A new monitoring technique has been devised which enables individual larvae to be screened for pyrethroid/endosulfan resistance directly from the field without changing their genetic makeup. The technique involves screening larvae, collected as eggs in the field, with a discriminating dose of either fenvalerate or endosulfan. This new technique has proved quite sensitive in detecting even smal l di fferences in resistance levels. It has also avoided the long delay and altered genetic makeup associated with the classical Fi bioassay method.