Changes in the conformation of duplex T4 DNAs embedded in poly(acrylamide) (PAAm) gels (as opposed to in an aqueous environment) were studied using fluorescence microscopy for the observation of individual molecules. In aqueous solutions, individual DNA chains exhibited thermal fluctuation, i.e., intramolecular and translational Brownian motion. With the addition of acetone, DNAs were adsorbed onto the surface of a glass plate accompanied by a discrete change in conformation from an elongated coil into a collapsed globule, while "metastable" coiled DNAs were present in the buffer solution with a lifetime of several days. In contrast, in the gel phase, embedded DNAs did not exhibit thermal motion. With the addition of acetone, elongated-coil DNA underwent a sharp but continuous transition driven by the collapsing transition of the gel. The resulting compact DNA was shrunk less than the collapsed DNA in free aqueous solution. In addition, it is shown that elongation of the PAAm gel affects the orientation of the DNA chains.