Although the fate of spray residues under grove conditions is of importance to citrus growers, little is known about the effect of wind, rain, and sunlight on the efficiency of insecticidal deposits. For example, the application of parathion to citrus trees is often followed, or even interrupted, by rain. It has not been known if removal of the insecticidal deposit by rain reduced control of purple scale, Lepidosaphes beckii (Newm.) sufficiently to make respraying advisable. In 1952, a research program was initiated to obtain information on this problem. As the aim of this research was to determine what reduction in control of purple scale could be expected from rain falling at predetermined intervals after application of parathion, the use of natural rainfall was impractical. Again, the physical characteristics of rainfall could not be duplicated under grove conditions with the equipment available. It was assumed that removal of parathion residues from citrus trees by spraying them with water would be more nearly complete and effect scale control more than would a natural rain falling at the same interval after the application of the spray. Therefore, an attempt was made to wash all parathion from sprayed trees, with the idea that if such removal of residues did not reduce scale control, neither would reduction of residues by natural rain. All trees in three experiments were sprayed with 1.7 pounds of 15 percent parathion wettable powder and 5.0 pounds of wettable sulfur per 100 gallons of spray, except for one set of plots in Experiments II and III. These were sprayed with 5.0 pounds of wettable sulfur per 100 gallons of spray, the parathion being omitted. Wettable sulfur was used in all sprays for the control of rust mite, Phyllocoptruta oleivora (Ashm.). The interval between the application of parathion sprays and the removal of toxic residues by washing the trees with water was recorded for each plot. Plots sprayed with parathion but not washed were used as the standard or check. Washing was