Abstract Many proteins are known to undergo small ubiquitin-related modifier (SUMO) modification by an E1-, E2-, and E3-dependent ligation process. Recognition that protein inhibitor of activated signal transducers and activators of transcription (STATs) (PIAS) proteins are SUMO E3 ligases raised the possibility that STATs may also be regulated by SUMO modification. Consistent with this possibility, a SUMO-ylation consensus site (ΨKxE; Ψ indicates hydrophobic residue, and x indicates any residue) was identified in Stat1 (ie, 702IKTE705), but not in other STATs. Biochemical analysis confirmed that Stat1 K703 could be SUMO modified in vitro. Mutation of this critical lysine (ie, Stat1K703R) yielded a protein that, when expressed in Stat1–/– mouse embryonic fibroblasts (MEFs), exhibited enhanced DNA binding and nuclear retention. This was associated with modest changes in transcriptional and antiviral activity. However, mutation of the second critical residue in the SUMO consensus site, E705 (ie, Stat1E705A), yielded a protein with wild-type DNA binding, nuclear retention, and transcriptional and antiviral activity. Similar observations were made when these mutants were expressed in primary Stat1–/– macrophages. These observations suggest that although Stat1 can uniquely be SUMO-ylated in vitro, this modification is unlikely to play an important role in regulating Stat1 activity in vivo.