Experiments were made to determine if black scale found on citrus trees in a part of the San Gabriel Valley, California are more difficult to control than black scale found in other districts. Scale from the so-called resistant district and from districts outside this area were established on small potted seedling orange trees and fumigated with hydrocyanic acid simultaneously and under the same covering. It was concluded that the black scale from the so-called resistant district are a hardy strain more difficult to kill than those in other districts. This hardy strain may be called resistant but not immune. It was possible to kill them all by a high dosage of cyanide.