The distribution of aerobic chemoheterotrophic microorganisms within a 21 m3 section of deep subsurface rock was determined. Nineteen samples for microbiological analysis were aseptically taken by hand from the walls of a 400 m deep subsurface tunnel after an alpine miner created fresh rock faces 0.76, 1.52, 2.28, and 3.04 m into the tunnel wall. The direct counts were several orders of magnitude greater than viable counts in all samples. One of each morphologically distinct bacterial type from each sample was purified and analyzed for fatty acid methyl esters (FAME) using the Microbial Identification System (MIDI). Numbers of bacterial types, diversity, and equitability of recoverable microbial communities were the same or similar using either morphotype or FAME analyses as the basis for distinguishing between bacterial types. Twenty-nine genera (Euclidean distance of [Symbol: see text]25) were found within the rock section, while 28 of the 210 bacterial types isolated were nonculturable under the growth regime required for cluster analysis. Most isolates clustered at the genus level with Arthrobacter, Gordona, and Acinetobacter. Two genera, containing 16 isolates, were unmatched to known organisms within the MIDI data base and clustered with other isolates at a Euclidean distance greater than 50. While some species (Euclidean distance [Symbol: see text]10) were recovered from multiple sites within the rock section, most were found at 1-3 sites and usually without a definitive pattern of distribution.