Spinosad is an insect control agent. Spinosad is derived from the metabolites of the naturally occurring bacteria, Saccharopolyspora spinosa. The objective of this study is to evaluate the toxicity as well as the latent efficacy of spinosad against the 4'11 instar larvae of the cotton leafworm. Spodopfera ifitoralfs (Boisd) Different bioassays namely; dipping, contact to a surface film and immersion techniques were used in this investigation. The biocide was bioassayed after 24, 48, 72 and 96 hours. The highest toxicity of spinosad was noticed after 96 hours. Results illustrated the superiority of dipping technique for the 4''1 instar larvae to the biocidal action of spinosad. The efficiency of the bioassay techniques reflecting the larval susceptibility could be descendingly arranged as follows: dipping. immersion and contact to a surface film. The corresponding Lcso and LCgo values against the tested larvae after 96 hours were 122.85 8. 518.61; 129.90 8. 606.78 and 141.34 8. 978.38 ppm. respectively. The 4'11 instar larvae of S. iiioraiis showed moderately susceptibility to the biocidal action of spinosad as demonstrated by the toxicity ratios. The toxicity ratios of all bioassay techniques were less than 1.00. The susceptibility index as well as the potency level expressed as numbers of folds at both L050 and Lean values increased with increasing the period of determination. The latent effects of spinosad on the pupation as well as the adult emergence were determined. The corresponding ECso and ECgo values associated to quintals scoring of pupation due to dipping, immersion and contact to a surface film of bioassays were 36.248. 251.95; 50.02 8 549.34 and 63.57 8. 711.97 ppm. respectively. Whereas the corresponding ICso and lovalues associated to inhibition of the adult mergence were 7.28 8. 215.44; 43.52 8. 291.42 and 54.12 8. 386.07 ppm, respectively.