Bioassays were conducted to study comparative and selective toxicity of commercial formulations of the neonicotinoid insecticides acetamiprid (Mospilan 20% SP) and imidacloprid (Imaxi 35% SC) against Bemisia tabaci (Genn.) and Spodoptera littoralis (Boisd.) using leaf dipping technique as well as Apis mellifera L. by orally mixing with food media. Based on the obtained LC₅₀ values, acetamiprid was more toxic to the tested adult and nymphal stages of B. tabaci than imidacloprid. The adult stage was more susceptible than nymph. Acetamiprid showed higher toxicity to eggs, the 2^nd and 4^th instar larvae of S. littoralis than imidacloprid. Moreover, eggs were more susceptible to the two tested insecticides than the larval instars, and the 4^th instar larvae were the least susceptible. Acetamiprid applied orally to honey bee workers at a field rate (50 µg a.i./ml) caused 3.33 % mortality with no symptoms of toxicity 24 hr. post treatment. The field rate of imidacloprid (265 µg a.i./ml) gave 90% mortality and obvious symptoms of poisoning were recorded 10 minutes after exposure. The LC₅₀ value for imidacloprid was 59.83 µg a.i./ml. The results revealed that the cyano-substituted compound (acetamiprid) was more toxic to the tested insect pests than the nitro-substituted compound (imidacloprid) and the later was highly toxic to honey bee. So, the efficiency and selectivity of neonicotinoid insecticides are related to the chemical group of the compound as well as the species and developmental stage of the insect.