We determined effects of acidification on sediment aerobic heterotrophic exo-amylase and exo-protease producing microbial populations in addition to effects on the total microbial population. Acidification of microcosms to sediment pHs of 6, 5, 4, and 3 followed by plate counts showed that numbers of total aerobic heterotrophic microbes, as well as numbers exo-amylase producers and exo-protease producers, decreased at pH 3. It appears that pH 3 may represent a critical level of damage. Numbers of total aerobic heterotrophs also decreased at pH 4. Numbers of all groups increased at pH 6. An accompanying change in species composition was observed with acidification, in which a type of microbe with little production of exo-amylase or exo-protease became dominant in the microbial community as pH decreased. Analysis of three metals (Al, Pb, Cu) indicated an increased mobilization of metals from the sediments at low pHs.