The identification and concentration of asbestos fibers in tap drinking water supplied in a central area of Akita Prefecture, Japan, were determined by phase-contrast microscopy and a scanning electron microscope equipped with an energy-dispersive X-ray microanalyzer. The following results were obtained. 1. Asbestos fibers were found in the tap water from two areas in which an asbestos-cement pipe was used for public water supply. The concentrations of asbestos fibers in the tap water were 2.7 x 10(4) to 27.0 x 10(4) fibers per liter of water in area A and 10.0 x 10(4) to 21.0 x 10(4) in area B. On the other hand, no asbestos fiber contamination was observed in tap water of area C, which shared a common water source with area A. A vinyl chloride pipe was used over the entire length of the water supply in route C. 2. Crocidolite was the predominant type of asbestos fiber detected in the tap water. Chrysotile and a mixture of chrysotile and amosite were also observed. 3. Almost all asbestos fibers detected in the tap water possessed the form of thick or sheaved fibers with lengths ranging from ca. 5 to 10 microns. Their shapes were very different from those of asbestos fibers found in the atmosphere. The typical form of the latter is short (ca. 1 micron in length) and needle-like. 4. It was suggested that the contamination of asbestos fibers in the tap water was caused by erosion and peeling off of the inner wall of the asbestos-cement pipe used as a conduit. In order to evaluate the safety of drinking water in Japan, an extensive survey on asbestos-fiber contamination in tap water is necessary.