The reasoning of adolescents in indeterminate situations was examined with particular attention given to how individuals incorporate the possibility that outcomes are determined jointly by more than one variable. Kuhn and Brannock's (1977) plant problem and two other similarly structured problems were administered to 25 inner-city adolescents ranging in age from 15 to 17 years. Each problem consisted of four cases for which the values of treatment variables and outcomes were described in detail. For each problem, subjects were asked to consider a fifth case, predict the outcome, and justify the prediction. In addition, subjects were questioned on the relevance of a variable which did not covary with outcome and the original procedure of the plant problem was modified by the introduction of a countersuggestion probe. In 73 percent of the overall justifications, subjects avoided the assumption that one variable independently determined the outcome. Five types of justification were found, and four of these demonstrated a concern with the determination of outcomes jointly by more than one variable. In particular, subjects supplemented their comparison and record arguments with covariation arguments. Implications for science education are briefly suggested. (RH)