The Arabidopsis thaliana ortholog of the 30-kD subunit of the mammalian Cleavage and Polyadenylation Specificity Factor (CPSF30) has been implicated in the responses of plants to oxidative stress, suggesting a role for alternative polyadenylation. To better understand this, poly(A) site choice was studied in a mutant (oxt6) deficient in CPSF30 expression using a genome-scale approach. The results indicate that poly(A) site choice in a large majority of Arabidopsis genes is altered in the oxt6 mutant. A number of poly(A) sites were identified that are seen only in the wild type or oxt6 mutant. Interestingly, putative polyadenylation signals associated with sites that are seen only in the oxt6 mutant are decidedly different from the canonical plant polyadenylation signal, lacking the characteristic A-rich near-upstream element (where AAUAAA can be found); this suggests that CPSF30 functions in the handling of the near-upstream element. The sets of genes that possess sites seen only in the wild type or mutant were enriched for those involved in stress and defense responses, a result consistent with the properties of the oxt6 mutant. Taken together, these studies provide new insights into the mechanisms and consequences of CPSF30-mediated alternative polyadenylation.