Abstract
Different aspects of the terms strain, clone and species are discussed. The term strain is commonly used to denote a pure culture here called the strain in the taxonomic sense but does also refer to a natural concept closely related to the clone. The term clone on the other hand is used both in a general and in a more restricted sense, the latter indicating a low degree of genetic exchange. The important distinction between the definition of a species and the criteria for a species is emphasised and the main kinds of criteria are considered.