To elucidate the function of Omega class glutathione transferases (GSTs) (EC 2.5.1.18) in multicellular organisms, the GSTO-1 from Caenorhabditis elegans (GSTO-1; C29E4.7) was investigated. Disc diffusion assays using Escherichia coli overexpressing GSTO-1 provided a test of resistance to long-term exposure under oxidative stress. After affinity purification, the recombinant GSTO-1 had minimal catalytic activity toward classic GST substrates but displayed significant thiol oxidoreductase and dehydroascorbate reductase activity. Microinjection of the GSTO-1-promoter green fluorescent protein construct and immunolocalization by electron microscopy localized the protein exclusively in the intestine of all postembryonic stages of C. elegans. Deletion analysis identified an approximately 300-nucleotide sequence upstream of the ATG start site necessary for GSTO-1 expression. Site-specific mutagenesis of a GATA transcription factor binding motif in the minimal promoter led to the loss of reporter expression. Similarly, RNA interference (RNAi) of Elt-2 indicated the involvement of this gut-specific transcription factor in GSTO-1 expression. Transcriptional up-regulation under stress conditions of GSTO-1 was confirmed by analyzing promoter-reporter constructs in transgenic C. elegans strains. To investigate the function of GSTO-1 in vivo, transgenic animals overexpressing GSTO-1 were generated exhibiting an increased resistance to juglone-, paraquat-, and cumene hydroperoxide-induced oxidative stress. Specific silencing of the GSTO-1 by RNAi created worms with an increased sensitivity to several prooxidants, arsenite, and heat shock. We conclude that the stress-responsive GSTO-1 plays a key role in counteracting environmental stress.