This paper describes the coexistence of two systems for classifying organisms and species: a dominant genetic system and an older naturalist system. The former classifies species and traces their evolution on the basis of genetic characteristics, while the latter employs physiological characteristics. The coexistence of the classification systems does not lead to a conflict between them. Rather, the systems seem to co-exist in different configurations, through which they are complementary, contradictory and inclusive in different situations-sometimes simultaneously. The systems come into conflict only through the researchers’ verbal articulations; in their application conflict is hardly present at all. This paper treats their relationship as the ’central tension of science’ in reverse. Rather than comprising heterogeneous communities that need boundary objects to make cooperation and integration possible, the field of molecular biology seems to be overwhelmingly homogeneous, and in need of heterogeneity and conflict to add drive and momentum to the work being carried out. The paper is based on observations of daily life in a molecular microbiology laboratory at the Technical University of Denmark. It is thus a ’real time’ and material study of scientific paradigms and discourses.