Gene expression profiling is a powerful new end point for ecotoxicology and a means for bringing the genomics revolution to this field. We review the usefulness of gene expression profiling as an end point in ecotoxicology and describe methods for applying this approach to non-model organisms. Since genomes contain thousands of genes representing hundreds of pathways, it is possible to identify toxicant-specific responses from this wide array of possibilities. Stressor-specific signatures in gene expression profiles can be used to diagnose which stressors are impacting populations in the field. Screening for stress-induced genes requires special techniques in organisms without sequenced genomes. These techniques include differential display polymerase chain reaction (DD PCR), suppressive subtractive hybridization PCR (SSH PCR), and representational difference analysis. Gene expression profiling in model organisms like yeast has identified hundreds of genes that are up-regulated in response to various stressors, including several that are well characterized (e.g., hsp78, metallothionein, superoxide dismutase). Using consensus PCR primers from several animal sequences, it is possible to amplify some of these well characterized stress-induced genes from organisms of interest in ecotoxicology. We describe how several stress-induced genes can be grouped into cDNA arrays for rapidly screening samples.