The persistence of water based (e.c.) and oil based (u.l.v.) formulations of endosulfan was compared by chemical and bio‐assay. A rapid rate of decay occurred with the e.c. formulation in the first 24 hours after which it declined to a rate similar to that followed by the u.l.v. formulation from the outset. The higher initial loss with the e.c. product resulted in about half the leaf residue in comparison to the u.l.v. formulation at a given point in time, with consequently more prolonged control ofHeliothislarvae with the u.l.v. formulation.