The conclusion is that Dursban emulsion or suspension formulations are not acceptable for use as a premises larvicide treatment in the control of Ae. aegypti under the conditions now present in the United States. It is conceivable that Dursban could be used as a graular formulation, at a much lower dosage as described for a non-residual type of larviciding or at an intermittent or less frequent interval without any effect on cholinesterase levels. The findings also suggest that the use of Dursban in any operation similar to that described previously might produce a depression of cholinesterase levels. Therefore, on all such control programs, careful monitoring of the cholinesterase levels of spraymen should be included in the operation.