A study was conducted to evaluate the impact of washing on the decrease in the effectiveness of the Long Lasting Impregnated Bed Net = IBN (deltamethrin) at a dosage of 55 mg/m2 on polyester 100%. The study was conducted by bioassay test on a sample 20 x 20 cm2 (untreated mosquito net, in the laboratory) of the IBN (deltamethin) and a control bed net (20 x 20 cm2); using malaria vectors Anophelesmaculatus and Anopheles aconitus. The bioassay test was conducted every day (4 replicates) with 10-15 mosquitoes/cone. The test was conducted until a mortality of 70% was reached. The exposure time was 3 minutes (recording knockdown mosquitoes), holding for 24 hours and counting the dead mosquitoes after 24 hours. Before the next bioassay test, the bed net sample was washed 3 times using 3 gram solid/hardsoap, and shaken on a shaker. Results show that the residual effect of the IBN decrease slowly after washing which has lost its effectiveness after 45 washings (Mortality< 70%).