An introduction to, and the Action Statement Microbial Diversity 21 arising from, the joint IUBS/IUMS workshop on Biodiversity amongst microorganisms and its relevance held in Amsterdam on 7-8 September 1991 are presented. The workshop was held in support of the IUBS/SCOPE/UNESCO initiative on the importance of biodiversity in ecosystem function, named Diversitas. The current state of knowledge on diverse aspects of the biodiversity of the various microorganism groups was reviewed by leading specialists, and a 14-point Action Plan was produced as a framework for future international initiatives in biodiversity research on microorganisms.