The primary purpose of the present study was to test the hypothesis that two general developmentally based levels of hypothesis-testing skills exist. The first hypothesized level presumably involves skills associated with testing hypotheses about observable causal agents; the second presumably involves skills associated with testing hypotheses involving unobservable entities. To test this hypothesis, a hypothesis-testing skills test was developed and administered to a large sample of college students both at the start and at the end of a biology course in which several hypotheses at each level were generated and tested. The predicted positive relationship between level of hypothesis-testing skill and performance on a transfer problem involving the test of a hypothesis involving unobservable entities was found. The predicted positive relationship between level of hypothesis-testing skill and course performance was also found. Both theoretical and practical implications of the findings are discussed. Following the review of several years of research into problem-solving performance, Perkins and Salomon (1989) concluded that, although expert performance manifests itself in contextualized ways, general cognitive skills (i.e., "habits of mind") exist. These general cognitive skills reveal themselves primarily as strategies of looking for counterexamples to test causal knowledge claims. Although Perkins and Salomon discussed such strategies as thinking tools of the philosopher, scientists recognize them as components of a scientific method that has as its core the generation and test of alternative hypotheses (cf. Baker & Allen, 1977;Burmester, 1952;Carey, 1998;Chamberlain, 1965;Lawson, 1995;Lewis, 1988;Moore, 1993; Platt, 1964). Essentially, this method embodies a set of generally applicable questions that must be raised and satisfactorily answered before drawing a firm conclusion about the relative truth or falsity of any particular causal claim. The set of questions reads more or less like this: What is the central causal question raised in this particular context? In addition to the proposed cause, what al-JOURNAL