History and Objectives: First step in chemical control of arthropod is assessment of their sensitivity to insecticides the present study is designed to determine the extent of sensitivity of cockroaches obtained from Shaheed Beheshti, Naghavi and Shabihkhani hospitals in Kashan to phosphorous, pyrothroid and carbamate insecticides in 1995-96.

Materials and Methods: Cockroaches were obtained by hand and trap. After 48 hours of in the laboratory, they were tested on the basis of WHO directives and by contact method and knock down experiments. Regression lines were calculated for KT50 and KT90 and level sensitivity was compared.

Results: The effect of Actlic insecticide (0.12 g/m²) on American cockroaches gathered from Naghavi and Shabihkhani compared to Shaheed Beheshti showed resistance ratio of 1.06 and 1.07 respectively. The effect of Sulfac insecticide (0.2 g/m²) on cockroaches obtained from Naghavi and Shabihkhani compared to the Shaheed Beheshti hospital showed resistance ratio of 2.3 and 2.1 respectively. Similar comparison of Coopex insecticide (0.125 g/m²) showed ratio of resistance of 3.04 and 1.64 respectively. This comparison for Ficom insecticide (0.24 g/m²) gave 1.25 and 1.18 ratio resistance for Naghavi and Shabihkhani hospitals compared to Shaheed Beheshti hospital. One hundred percent mortality and morbidity of species under examination was obtained for all insecticide after 24 hours of contact.

Conclusion: It seems the repeated use of insecticides in Naghavi and Shabihkhani hospitals, which compared to the Shaheed Beheshti hospital are older has increased the level of tolerance of cockroaches considerably. Insects at Naghavi and Shabihkhani have been under stress and overuse of insecticides and have developed tolerance of some insecticides.