Abstract: Mortality LD50 values of a series of 37 pyrethroidal esters have been previously measured against a susceptible strain of housefly (Musca domestica). Structural features of these compounds were related to the LD50 values by multiple linear regression (MLR) to see their influence on mortality. Pyrethroidal esters structures were first built by the Marwin Sketch (Marwin Sketch 6.0, 2013, ChemAxon, http://www.chemaxon.com) software and than conformational analysis was perfomed by the OMEGA (Omega v.2.4.6 (OpenEye Scientific Software, Santa Fe, NM., http://www.eyesopen.com, 2010) program. Structural descriptors were calculated for the minimum energy structures by the Dragon (Dragon Professional 5.5/2007, Talete S.R.L., Milano, Italy) and EPI Suite™ (US EPA. [2012]. Estimation Programs Interface Suite™ for Microsoft® Windows, v. 4.11. United States Environmental Protection Agency, Washington, DC, USA.) software. Instant JChem (Instant JChem 5.11.3, 2012, ChemAxon, http://www.chemaxon.com) was used for structure database management, search and prediction. Genetic algorithm was applied for descriptor selection. The dataset was divided in training and a random test set. MLR analysis has been applied after variable selection carried out by the genetic algorithm included in the QSARINS v. 1.1 program, using the RQK fitness function, with leave-one-out cross-validation correlation coefficient as constrained function to be optimized. Structural features which influence insecticide toxicity were discussed.