Tax, the virally encoded activator of the human T-cell leukemia virus type 1 long terminal repeats, regulates the expression of many cellular genes. This protein has been implicated in transformation events leading to the development of adult T-cell leukemia. Because subcellular localization contributes importantly to protein function, we determined the compartment(s) within the cell in which Tax is found. Using confocal microscopy, we found that Tax localizes to subnuclear domains which overlap with structures previously identified as interchromatin granules or spliceosomal speckles. These Tax speckled structures are coincident with a subset of nuclear transcriptional hot spots. Disruption of the Tax speckled structures by heat shock revealed the existence of different populations of Tax. One population of Tax is tightly associated with nuclear speckles. A second population exists outside of the speckles and is transcriptionally active for some promoters.