Microbial species diversity was determined for the acidogenic populations of four different digesters treating landfill leachate, a petrochemical effluent and sewage. The digesters were a downflow fixed-bed, conventional, cold conventional and an upflow hybrid design. The 288 strains isolated from the digesters were characterized using API test systems and identifications confirmed using current taxonomic methods. The taxonomic diversity was determined using the Shannon (H′) and equitability (J′) indices. The species from the four digesters were very similar with the exception of those from the digester treating the landfill leachate. The higher diversity in the latter was probably due to the diverse composition of the leachate, suggesting that members of this population were more generalists, as opposed to the more narrowly constrained members in the other three digesters. The data indicated that in a digester environment dominated by a more specific carbon containing effluent, like a petrochemical effluent, the population becomes stenotolerant and is characterized by a lower species diversity. Such a population would not be able to cope as well with environmental fluctuations. With a more diverse substrate like the landfill leachate, generalists, having a wider range for environmental variables, would dominate. This is important in terms of anaerobic digester process efficiency and stability.