PHILADELPHIA -- University of Pennsylvania professors David Cass and Michael L. Klein are among 187 Americans elected this month as fellows of the American Academy of Arts and Sciences.

Election to the Academy is a highly competitive process that recognizes significant contributions in all scholarly fields and professions. The Academy will welcome this year new fellows Oct. 11 at its annual induction ceremony in Cambridge, Mass.

Cass, professor of economics and director of the Center for Analytic Research in Economics and the Social Sciences, was cited by the Academy for research that deals with he careful refinement and extension of neoclassical theory, especially the specification of utility functions and conditions for existence of general equilibrium.

Cass, whose research interests include pure theory of capital, individual behavior under uncertainty and models of financial equilibrium, has investigated the xistence of equilibrium under conditions of overlapping generations, important for growth theory and under conditions of uncertainty.

He has been honored with a Guggenheim Fellowship in 1970-1971 and a Distinguished Fellowship in the American Economic Association, the most prestigious U.S. honor in the field of economics.

Cass joined the Penn faculty in 1974 after four years as an economics professor at Carnegie Mellon University and five years as an economics professor at Yale University. He received an A.B. in 1958 from the University of Oregon and Ph.D. from Stanford University in 1965.

Klein, professor of chemistry and physical sciences and director of Penn Laboratory for Research on the Structure of Matter, was cited by the Academy for work that as led to physically significant and predictive descriptions of hydrogen-bonded liquids, self-assembled monolayers, supercooled liquids, conducting fluids and biological membranes.Klein has devised computational methods to predict how the properties of matter respond to changes in pressure and temperature and is noted for his computer simulations of molecular materials.

Klein, who has authored approximately 500 papers in research journals, ranks as the world 96th-most-cited chemist, according to an Institute for Scientific Information analysis of research papers published from 1981 to 1997. He has edited three books and serves on the editorial boards of numerous journals. He was a Guggenheim Fellow in 1989-1990 and is a fellow of the Royal Society of Canada, the Chemical Institute of Canada and the American Physical Society.

Klein joined the Penn faculty in 1987 after 19 years at the National Research Council Canada, culminating as principal research officer in NRCC Chemistry Division. He received a B.Sc. in 1961 and Ph.D. in 1964 from the University of Bristol in the U.K.

Founded in 1780 by John Adams, John Hancock and other scholar-patriots, the American Academy of Arts and Sciences draws upon the wide-ranging expertise of its membership to conduct innovative, non-partisan studies on international security, social policy, education and the humanities. The current membership of some 3,700 includes more than 150 Nobel laureates and 50 Pulitzer Prize winners.