The susceptibility of two field strains of the cotton aphid, Aphis gossypii Glover collected from Fayoum and Gharbia Governorates in Egypt representing Upper and Lower Egypt during 2005-2007 to eight organophosphorus and four carbamate insecticides was determined using a slide-dip technique. The results indicated that both strains showed very low levels of resistance to chlorpyrifos methyl, chlorpyrifos ethyl and carbosulfan. In contract, resistance to profenofos, dimethoate, cyanofos, and thiodicarb was very high. Moderate to relative high resistance was observed for pirimiphos methyl, malathion, prothiophose, methomyl and pirimicarb. Generally, Gharbia strain exhibited higher resistance to profenofos, dimethoate and malathion than Fayoum strain, while the resistance level of prothiophose in Fayoum strain was higher than in Gharbia strain, but no significant difference in levels of resistance between the two strains was observed in the other insecticides. The toxicity index of tested insecticides against the two field strains was also determined during 2005-2007. With regard to organophosphorus compounds, chlorpyrifos methyl was the most toxic action followed by chlorpyrifos ethyl and then pirimiphos methyl, while the other insecticides were least toxic. As for the carbamates, carbosulfan showed the most toxic effect followed by methomyl, while thiodicarb and pirimicarb were the least in both strains.  As a general conclusion, the OP_S chlorpyrifos methyl and chlorpyrifos ethyl and the carbamate carbosulfan which showed very low levels of resistance are preferred pesticides for controlling cotton aphid.