The crown gall teratoma tumor line BT37, incited by Agrobacterium tumefaciens strain T37, has been found to contain part of the tumor-inducing plasmid, pTi T37, of the inciting strain. This foreign DNA segment, called T-DNA, is maintained at several copies per diploid tumor cell. We have examined subcellular DNA fractions from this tumor line in an effort to determine whether T-DNA is in chloroplasts, mitochondria, or nuclei. Tumor cell chloroplast DNA exhibited EcoRI and Bst I endonuclease cleavage patterns identical to those of normal tobacco chloroplast DNA. Tumor cell mitochondrial DNA exhibited a complex Bst I cleavage pattern that did not differ detectably from that of normal tobacco mitochondrial DNA. Southern blots of tumor chloroplast and mitochondrial cleavage products did not hybridize with labeled pTi T37 DNA, whereas blots of tumor cell nuclear DNA cleavage products hybridized strongly. We conclude that T-DNA is located not in chloroplasts or mitochondria but rather in the nuclear fraction of this tumor line.