The study of "scientific excellence" is taking on increasing importance in the development of research policies in many nations. However, scientific excellence is difficult to define because of its multidimensional and highly complex character. This work contributes to the state of the art by exploring an effective, simple, and inexpensive bibliometric methodology that further identifies "excellent" centers of research by beginning with the individual researchers affiliated with such centers. The study concentrates on public research organizations in Italy, analyzing 109 scientific categories of research in the "hard" sciences and 157 centers of excellence operating in 60 of these categories. Findings from this first application of the methodology should be considered exploratory and indicative. With a longer period of observation and the addition of further measurements, this methodology could be extended and adapted to a variety of national and supranational contexts, aiding policy decisions at various levels. (Contains 10 tables, 4 figures and 9 notes.)