Adult males and females of a strain of Blattella germanica cultured from collections made in the School of Medicine, San Juan, P.R., were tested for insecticide resistance-susceptibility. According to the LT50 test these cockroaches demonstrated resistance to chlordane (â™, and ♀ LT50 > 72), lindane (â™, LT50 = 7); ♀, 6), and dieldrin (â™, and ♀ LT50 > 72), but were susceptible to malathion (â™, LT50 = 1.75; ♀, 1.25). Lethal concentrations at the 50- and 90-percent levels, estimated on the basis of combined counts of "dead" and "moribund" insects, indicated that the cockroaches are susceptible to the following insecticides, listed in decreasing order of effectiveness: Bayer 29493 (â™, LC50 = 0.007; ♀, 0.012; â™, LC90 = 0.016; ♀, 0.024), diazinon (â™, LC50 = 0.02; ♀, 0.01; â™, LC90 = 0.05; ♀, 0.02), synergized malathion (â™, LC50 = 0.07; ♀, 0.1; â™, LC90 = 0.12; ♀, 0.15), and malathion (â™, LC50 = 0.13; ♀, 0.08; â™, LC90 = 0.35; ♀, 0.2).Pruebas de resistencia y susceptibilidad a insecticidas se llevaron a cabo en adultos machos y hembras de una cepa de Blattella germanica criados en el laboratorio, de especímenes recogidos en la Escuela de Medicina en San Juan, Puerto Rico. Estas cucarachas, de acuerdo con la prueba de LT50, demostraron tener resistencia a clordano ((â™, y ♀ LT50 > 72), lindano (â™,LT50 = 7; ♀, 6), y dieldrin (â™,and ♀ LT50 > 72), pero ser susceptibles amalathion (â™,LT50 = 1.75; ♀, 1.25). Concentraciones letales a un nivel de 50 a 90 por ciento, calculadas a base de contajes combinados de especímenes "muertos" y "moribundos," demostraron que las cucarachas son susceptibles a los siguientes insecticidas, colocados en orden descendente de efectividad: Bayer 29493 (â™,LC50 = 0.007; ♀, 0.012; â™, LC90 = 0.016; ♀, 0.024), diazinon (â™,LC50 = 0.02; ♀, 0.01; â™, LC90 = 0.05; ♀, 0.02), synergized malathion (â™,LC50 = 0.07; ♀, 0.1; â™, LC90 = 0.12; ♀, 0.15), y malathion (â™,LC50 = 0.13; ♀, 0.08; â™, LC90 = 0.35; ♀, 0.2).