The impacts of the 30 active substances, most used in Costa Rica (reference year 1998), are evaluated using two models originally developed to support comparative assertions in the context of the life cycle assessment: the multimedia fate, exposure and effect model Impact Assessment of Chemical Toxics 2002 (IMPACT 2002), and the Dynamic Model for Pesticides Residues in Plants. Results show that only five active substances - namely diazinon, mancozeb, chlorothalonil, terbutylazine and ethoprophos - contribute to more than 75% of the aquatic ecotoxicity and yet represent less than 40% of the amount used. More than 90% of the human toxicity is generated by only two active substances - namely terbufos and chlorothalonil - whereas they represent less than 10% of the total amount used. Overall, it would be possible to achieve a 90% reduction of human toxicity and a 75% reduction of aquatic ecotoxicity due to pesticide used in Costa Rica, focusing on only a few active substances. For instance, propiconazole or tridemorph could potentially be replaced by chlorothalonil; ametryn, diuron or paraquat by glyphosate; terbufos by cypermethrin; methyl bromide by metam sodium. (c) 2006 Elsevier B.V. All rights reserved.