The Ontology* of Microbial Phenotypes (OMP) is being developed to standardize capture of phenotypic information, including both processes and physical characteristics, from microbes. The OMP team comprises ontologists, microbiologists, and annotators. and ontology development is being performed in conjunction with the development of a wiki designed for annotation capture. Term development is being guided by following, to as great an extent as possible, the structure of existing ontologies. All OMP terms have Aristotelian definitions, and. when appropriate, they have genus-differentia cross products composed of terms from external ontologies. Initially, OMP is being used to annotate the prokaryotic model organism Escherichia coli. Eventually we anticipate that diverse user groups will employ OMP for standardized annotation of various microbial phenotypes. much in the same way that, the Gene Ontology has standardized the annotation of gene products. Definitions of phenotypes and links to the original literature will facilitate the experimental characterization of phenotypes.