The record of Archaean microfossils is sparse(1). Of the few bona fide fossil assemblages, most are from shallow-water settings, and they are typically associated with laminated, stromatolitic sedimentary rocks(2-4). Microfossils from deep-sea hydrothermal systems have not been reported in Precambrian rocks (>544 million years old), although thermophilic microbes are ubiquitous in modern sea-floor hydrothermal settings(5,6), and apparently have the most ancient lineages(7,8). Here, I report the discovery of pyritic filaments, the probable fossil remains of thread-like microorganisms, in a 3,235-million-year-old deep-sea volcanogenic massive sulphide deposit from the Pilbara Craton of Australia. From their mode of occurrence, the micro-organisms were probably thermophilic chemotropic prokaryotes, which inhabited sub-sea-floor hydrothermal environments. They represent the first fossil evidence for microbial life in a Precambrian submarine thermal spring system, and extend the known range of submarine hydrothermal biota by more than 2,700 million years(9). Such environments may have hosted the first living systems on Earth, consistent with proposals for a thermophilic origin of life(10-13).