A model for the development of scientific reasoning in adolescents was formulated largely upon the basis of Piagetian theory. Included as potential determinants of scientific reasoning were experience, age, locus of control, field independence-dependence (FID), rigidity/flexibility, intelligence quotient (IQ), and sex. Causal relationships between these variables were hypothesized a priori with strong theoretical, heuristic, and empirical support. Data collected from middle school (N=106) and high school (N=96) students were used to test the hypothesized model by path analyses. Multiple regressions were performed according to path analytic methods to acquire standardized beta weights for determinant variables. These beta values were used as path coefficients for each of the posited relationships. Significant path coefficients resulted for these variables and scientific reasoning: age; IQ; FID; and experience. Age and IQ were stronger determinants of scientific reasoning than were FID and experience. Indirect effects of locus of control on scientific reasoning through FID also was obtained. Paths involving sex and rigidity/flexibility were not significant. The revised model included significant paths which explained 61 percent of the variance in scientific reasoning. (Author/JN)