Abstract: Residual spraying to control Phlebotomus chinensis with deltamethrin was applied to all field caves and 13-14 villages within the radius of 10 km2 surrounding Yunyan town in 1985-1986. The dosage used was 25 mg/m2 in field caves and livestock caves and 12.5 mg/m2 in villages. The test area covered 74000 m2. In 1985, after spraying, sandfly density decreased to very low level until the end of sandfly season. However, it was still very high in unsprayed field caves throughout the sandfly season.Three and a half months after spraying, sandfly specimens were collected and forced to contact the intact sprayed wall surface. The first knock-down time was between 33 to 61. minutes. However, contact with poorly sprayed wall surface gave a longer knock-down time between 75 and 305 minutes.In 1986, after spraying, the sandfly densities were markedly decreased as compared with the same months in the previous year. In June in the unsprayed area the sandfly density decreased by 79.1% in field caves and 71.5% in livestock caves of village. In August, the sandfly density decreased by 46.9% in the field caves and 59.6% in village area. The results showed that the residual potency effect of deltamethrin was long enough to suppress sandfly density throughout the sandfly season (3.5 to 4 months).Deltamethrin spraying (25 mg/m2) in field caves and livestock caves before or at the beginning of sandfly season, was effective and practical in the control of peri-wild P. chinensis in the loess plateau of northern Shaanxi.