Micrometre-sized particles resembling microorganisms have been found in liquid-rich fluid inclusions in hydrothermal quartz crystals from a research drill hole in Yellowstone National Park, Wyoming. The particles are moving, apparently in Brownian motion. The 1-2-mm-long quartz crystals were obtained from fractures in late Pleistocene rhyolite at depths of 59.5,102.1, and 102.5 m. The measured temperature and pressure at 102.1 m, where about 99% of the moving particles were found, were about 190 °C and 1.24 MPa, respectively. Homogenization temperatures (190–280 °C) of fluid inclusions at this depth mostly exceed measured temperatures, which suggests that the fluid inclusions probably did not form under present-day conditions. Instead, the fluid inclusions probably formed ⩾45 000–14 000 B.P. at a much higher pressure owing to the weight of at least 490 m of glacial ice. Only 3 of the more than 200 quartz crystals examined contain up to several hundred rodlike, threadlike, or irregular-shaped moving particles. Fluid inclusions in 17 other quartz crystals each contain one or two moving particles that were observed only because of their continuous Brownian motion.