1. A DNA ligase has been isolated from various organs and subcellular components of plants. It has been purified 600-fold from pea roots. 2. The plant ligase seals single nicks in double-stranded plant and phage T 7 DNA, resulting in the formation of typical 3′5′-phosphodiester linkages. The enzyme further catalyzes the conversion of hydrogen-bonded circles to covalent circles of phage λ DNA. 3. For activity, the enzyme obligatorily requires Mg 2+, ATP and duplex DNA substrates, and for optimum activity also a sulfhydryl compound. 4. The major subcellular concentrations of the enzyme are localized in nuclei and chloroplasts. Roots contain more enzyme activity per unit protein than leaves. The specific activity of etiolated leaves is higher than of green leaves. Roots of dwarf plants possess a lower specific activity than roots of normal-sized plants.