Popper has proposed a ?theory of situational rationality? as a basis for the social sciences. This theory of rational action is reconstructed and its methodological and substantial implications discussed. It is shown that methodologically Popper's idea of rational action leads to a version of theoretical instrumentalism which is incompatible with his general philosophy of science, and that substantially it implies an unacceptable theory of social institutions. Instrumentalism can be avoided by a more contentful theory of human action encompassing ?non?rational? or (...) ?irrational? kinds of action; Popper's theory of institutions might be improved through a more comprehensive theory of collective action. (shrink)