Homeobox genes contain sequences coding for DNA-binding motifs. These sequences are highly conserved across both the animal and plant kingdoms. Members of this gene family code for transcription factors that are key regulators of developmental organization. In an attempt to further elucidate the developmental process of tuberization in the potato plant, a full-length homeobox cDNA has been isolated via sequence homology from an early tuberization stage cDNA library constructed from 4-day axillary bud tubers. This cDNA, POTH1, has been sequenced and characterized by Southern blotting, northern analysis, sequence comparison, and in situ hybridization. POTH1 is shown to be a class I homeobox gene with 45% overall similarity to Kn-1 of maize and 73% match in the homeobox region. Messenger RNA accumulation studies indicate that POTH1 mRNA, unlike most homeobox transcripts, is not limited to a particular organ or developmental stage. Instead, POTH1 mRNA accumulates in rapidly growing cells of the potato plant: the apical meristems, the vascular cambium, the edges of young leaves, axillary buds, and root tips. In situ studies indicate accumulation of POTH1 mRNA in the tunica and corpus layers of the apical dome of the shoot apex and the stolon apex. In the stolon, growth and proliferation of the parenchymal cells associated with the vascular cambium contribute to swelling during early stages of tuberization, and this tissue accumulates POTH1 mRNA. It is possible that POTH1 may be posttranscriptionally regulated in a particular organ or stage of growth, or that it is involved in a wider range of growth processes than most plant homeobox genes.