ALTHOUGH around 70% of the Earth's surface is marine, little is known about the microbiology of underlying sediments, which can be more than a kilometre deep(1). Selective degradation of organic matter within sediments over geological time profoundly affects the chemical composition of the ocean and atmosphere(2). Microbial processes have a fundamental role in surface sediments(3,4), but despite geochemical evidence(5), their significance in deeper sediments has not been established(6). Here we report the discovery of viable sediment bacterial populations at five Pacific Ocean sites to depths >500 m. Bacterial distributions and activities are commensurate,vith geochemical changes. Bacterial profiles with depth are remarkably consistent, and deviations can be linked to specific environmental factors. The rate of decline in these populations indicates that bacteria are present to even greater depths. These bacteria, some of which are unique, must have a profound effect on deep-sediment diagenetic processes, and their presence considerably extends the biosphere.