Larvicidal efficacy of a new semi-synthetic avermectin derivative Methylamine avermectin (Radical 0.5% EC) was determined against larval instars of the Egyptian cotton leafworm, Spodoptera littoralis (Biosd.) in the laboratory, field and semi field experiments.  2^nd and 4^th instar larvae showed greatest susceptibility to the Radical in the laboratory experiment. The LC_50s values of the 2^nd and 4^th larval instar after 48 hours were 0.005 and 0.008 ppm, respectively. Radical was tested with recommended dosage (200 ml / 100 liter water) in field; it caused 84.6% reduction of pest population up to day 8 post-treatment. On the other hand, the semi field application of the same recommended dose on the 2^nd instar larvae showed general mean 73.6% mortality, 7 days after post-treatment. Also, some biochemical changed in the 4^th instar larvae after 48 hours of treatment with tested bioinsecticide were measured. It's clear from the results that activities of trehalase, invertase and acetylcholine esterase were increased in all treatments. Tested bioinsecticide reduced the activity of alkaline phosphatase at all doses compared to untreated larvae. No significant changes in acid-phosphataseactivities were observed at all treatment doses. On studying the effect of esterases isozymes patterns, there were no differences in number and position of esterases isozymes between untreated and treated larvae in the whole larval body tissues although each band different in its concentration. The toxicity of the formulation to some beneficial predators was also evaluated in the field. There was no detectable effect of these bioinsecticide on naturally occurring beneficial species.