Laboratory and field trials were carried out in the period 1948–52 with DDT, BHC, and other chlorinated hydrocarbon insecticides and with parathion for the control of mangold‐fly larvae (Pegomyia betaeCurtis) mining in the leaves of sugar beet. Kills ranging from 85 to 99% were obtained within 6 days with ‘high’‐ and ‘ low‐volume’ sprays using 6–24 oz. of DDT per acre in the form of an oil emulsion applied as a top cover only. Formulation in oil improved the action of the DDT, it being necessary to use dusts and wettable powders at excessive rates to obtain equivalent kills. DDT was usually superior to BHC when compared at customary rates of application for both insecticides. When oil emulsions were compared side by side, however, as ‘low‐volume’ sprays, DDT was not significantly better than BHC. In the same trial, toxaphene, aldrin and dieldrin also gave kills of the same order as BHC and DDT. With the possible exception of parathion, none of the insecticides used appeared to have any effect upon eggs.Plot yields of two of the trials were determined in the autumn, but only at one trial was there a significant increase in yield on the treated plots. At this trial the mangold‐fly infestation was severe, there being twenty‐five unhatched eggs and twenty larvae per plant at the time of treatment. Attempts were made to assess the degree of defoliation and to relate it to the larval population and to the effect on yield.Commercial insecticides of the ‘stock emulsion’ and ‘miscible oil’ types occasionally caused phytotoxicity when applied to sugar beet at excessive, ‘high‐volume’ rates and concentrations. More serious phytotoxicity was observed in tests of commercial insecticides used at ‘low‐volume’ rates and concentrations. Out of ten products tested, only three were acceptable over the whole range of rates and concentrations employed, whilst four caused injury over the whole range.The results obtained in the field and the laboratory were in agreement but did not always agree with the findings of other workers. BHC has previously been recommended largely on the basis of experiments on larvae mining very small beet plants when in, or just beyond, the cotyledon stage. All the trials by the authors were on sugar‐beet seedlings with about six foliage leaves and normal in size for the time of the year. On these plants BHC proved ineffective both as dust and ‘high‐volume’ spray when used at customary rates, but was effective as a ‘low‐volume’ spray in oil emulsion.The successful control of mining larvae by DDT and other chlorinated hydrocarbons is prob