Glutathione S-transferases (GSTs) play a central role in the detoxification of xenobiotic compounds including insecticides. Overexpression of GST is an important mechanism of insecticide resistance. The aim of this study is to investigate GST-based resistance to DDT in Aedes aegypti from Thailand. Putative GST genes were retrieved from the Aedes aegypti genome and the gene sequences were confirmed. Twenty seven GST genes were identified in Aedes aegypti. There were classified into at least 6 classes and their relationship to Anopheles gambiae GSTs investigated using phylogenetic analysis. Thirteen GSTs were further characterised; two were classified as Delta, two as Epsilon, one as Sigma, two as Theta class and three were unclassified. Both GSTdI and GSTsI are alternatively spliced. The expression of four GST transcripts, GSTe2, GSTe4, GSTu3 and GSTt1 was quantified. Only GSTe2 was up regulated in the DDT/permethrin resistant strain. Recombinant GSTE2-2 showed DDT dehydrochlorinase activity toward DDT. The expression of GSTe4, GSTu3 and GSTtI were not correlated with insecticide resistance. GSTu3 has been previously studied and implicated in DDT/permethrin resistance in Aedes aegypti from South America [Grant and Hammock (1992) Mol Gen Genetics 234: 169-176]. However, this gene was not over expressed in the resistant strain from Thailand. Recombinant GSTU3-3 was not able to metabolise DDT. Recombinant GSTE2-2 and GSTU3-3 showed an affinity to bind hematin, suggesting these enzymes act as binding protein, besides the enzymatic properties. The binding of hematin by GST may protect mosquitoes against heure toxicity during blood feeding