The increasing need for ontological support in the life sciences is being generally acknowledged, but up until now, the development of biological upper level ontologies have mainly been purpose-driven. An example is the GENIA ontology, covering biological continuants, which has mainly been devised for corpus annotation in a text mining context. As an alternative, we here present BioTop, an upper ontology of physical continuants in the domain of biology, with a coverage similar to the GENIA ontology. We report on design specifications and modeling decisions based upon formal ontology principles. A major desideratum was to describe the classes in terms of both necessary and sufficient properties. This could be accomplished for 85 out of 146 classes. Conscious of the trade-off between expressivity and computational feasibility, we opted for constraining our description language to OWL-DL, with the advantage of using a terminological classifier for maintaining consistency, as a major heuristic support in the ontology building process.