Trials with the parasiteEncarsia formosa to control whitefly (Trialeurodes vaporariorumWestw.) on cucumbers showed that the most predictable results were obtained by the “classical” method of establishing a low, even infestation of the pest on the crop artificially before introducing the parasite. Other methods, more acceptable to the grower are described including the regular and routine introduction of parasites from planting until parasitised scales are found in the crop which also usually gave satisfactory results.