Intraepithelial lymphocytes (IELs) are abundant, evolutionarily conserved T cells, commonly enriched in T cell receptor (TCR) gammadelta expression. However, their primary functional potential and constitutive activation state are incompletely understood. To address this, serial analysis of gene expression (SAGE) was applied to murine TCRgammadelta+ and TCRalphabeta+ intestinal IELs directly ex vivo, identifying 15,574 unique transcripts that collectively portray an "activated yet resting," Th1-skewed, cytolytic, and immunoregulatory phenotype applicable to multiple subsets of gut IELs. Expression of granzymes, Fas ligand, RANTES, prothymosin beta4, junB, RGS1, Btg1, and related molecules is high, whereas expression of conventional cytokines and high-affinity cytokine receptors is low. Differentially expressed genes readily identify heterogeneity among TCRalphabeta+ IELs, whereas differences between resident TCRgammadelta+ IELs and TCRalphabeta+ IELs are less obvious.