Laboratory experiments carried out on plants strongly suggested that volatilization is the most important process responsible for the short residual life of foliar applied chlorpyrifos. In order to control its volatility, chlorpyrifos was incorporated into a serious of materials including abietic acid, abietic acid esters, phenolic resin, bitumen, montan wax and Pinechem (a sodium hydroxide digested pine wood material). The rate of release of chlorpyrifos from these materials has been evaluated in the laboratory by using a microbalance held at constant vacuum. A range of release rates was observed showing large differences between the fastest (Pinechem and bitumen) and the slowest (phenolic resin and abietic acid esters). The matrices were converted into emulsions by addition of water, detergents and organic solvents. The chlorpyrifos containing emulsions were applied dropwise with a fine-needle syringe to a marigold plant and chlorpyrifos release rates from the treated plant were estimated by extracting the treated plant and by analysis of the residual chlorpyrifos. The matrix ranking was found to be the same as that obtained in the microbalance experiment. Biological tests of the emulsions involved observation of the insect mortality in dishes to which emulsion had been applied. The results showed that the performance of the chlorpyrifos was under certain circumstances greatly improved by using controlled release emulsions of chlorpyrifos having a slower release rate. The best controlled release formulation was an emulsion based on abietic acid esters. For this formulation the lowest dose required to achieve 100% mortality of the test insect species was less than a hundredth ofthat of the conventional formulation, for 24 h exposures of the insects to the formulations.