The results of a large number of studies on microorganisms subjected to various degrees of substrate limitation have led to the idea that many species are particularly well adapted to growth at a very low rate at extremely low nutrient concentrations. The possible similarity between this type of bacteria and oligotrophic species is discussed. Some attention is paid to the problem of predicting the competitiveness of microbial species. To this end the apparent specific affinity of an organism for a given substrate is discussed in some detail. It is attempted to bring terminology used in describing this parameter in line with that commonly used in microbial physiology and ecology. Using one particular field study as an example the possible usefulness and limitations of this concept in field studies are discussed.