The purpose of this descriptive study was to investigate the developmental patterns in logical reasoning of students in grades 6-10 over a span of 20 months. The Group Assessment of Logical Thinking (GALT) was administered to the sample (N=84) during the fall of 1986, the fall of 1987, and the spring of 1988. The GALT measures six reasoning modes: conservation; proportional reasoning; controlling variables; probabilistic reasoning; correlational reasoning; and combinatorial logic. Correlational reasoning was found to be the most difficult for the total sample for the three administrations of the GALT. The results of the two-tailed t-test for gender difference were not significant. An ANOVA (GALT score by grade level) was significant for each test administration. The results seem to indicate: (1) the majority of this sample was not functioning at the formal operational reasoning level; (2) a significant movement from concrete to transitional operational reasoning occurred at the end of grade seven; and (3) a plateau effect occurred between grades eight and nine. (MVL)