Introducing greater objectivity to selection of indicator taxa produces results that are likely to reduce uncertainty, be more efficiently obtained and more clearly communicated. Seven criteria are presented that can be used to objectively test the claim that a given taxon is an ideal indicator: (i) well known and stable taxonomy; (ii) well known natural history; (iii) readily surveyed and manipulated; (iv) higher taxa broadly distributed geographically and over a breadth of habitat types; (v) lower taxa specialized and sensitive to habitat changes; (vi) patterns of biodiversity reflected in other related and unrelated taxa; and (vii) potential economic importance. These criteria have different priorities depending on which of two general categories of biodiversity the indicator taxon is to be used. Monitoring places an emphasis on sensitivity to habitat change, and inventory places an emphasis on systematics. An index is suggested by which the results of selecting an indicator taxon can be more accurately communicated. This index is based on the number of criteria that are successfully tested for the proposed indicator and their priority.