The rates of microbial degradation of a variety of dissolved and particulate substrates in water and sediment from the Okenofee Swamp, Georgia, and Corkscrew Swamp, Florida, are compared. These two wetland ecosystems share many of the same types of plant communities and both are peat-forming systems. As in many wetlands, streams, and small lakes, vascular plant detritus is a major source of organic matter in these two freshwater swamps. The relationship between pH and rates of biodegradation or organic substrates was determined for natural microbial assemblages and for several bacterial isolates from these environments. Results suggest that microbial degradative processes that rely on extracellular enzymes are depressed at low pHs, whereas the microbial utilization of low-molecular-weight compounds that can be directly transported into cells is not substantially affected by variations in pH from 4 to 8. Furthermore, microbial populations will not 'adapt' for the rapid utilization of lignocellulosic substrates at low pHs.