Nuclease I enzymes are responsible for the degradation of RNA and single-stranded DNA during several plant growth and developmental processes, including senescence. However, in the case of senescence the corresponding genes have not been reported. We describe the identification and characterization of BFN1 of Arabidopsis, and demonstrate that it is a senescence-associated nuclease I gene. BFN1 nuclease shows high similarity to the sequence of a barley nuclease induced during germination and a zinnia (Zinnia elegans) nuclease induced during xylogenesis. In transgenic plants overexpressing the BFN1 cDNA, a nuclease activity of about 38 kD was detected on both RNase and DNase activity gels. Levels of BFN1 mRNA were extremely low or undetectable in roots, leaves, and stems. In contrast, relatively high BFN1 mRNA levels were detected in flowers and during leaf and stem senescence. BFN1 nuclease activity was also induced during leaf and stem senescence. The strong response of the BFN1 gene to senescence indicated that it would be an excellent tool with which to study the mechanisms of senescence induction, as well as the role of the BFN1 enzyme in senescence using reverse genetic approaches in Arabidopsis.