This study examined the effects of concept mapping on the science achievement of middle grade science students. The subjects were 182 eighth-grade students, distributed into eight intact earth science classes by ability levels. The ability level variable also was examined as a possible effect on student achievement. Two teachers were involved in teaching a unit on weather for nine weeks. An objective weather test and six performance assessment items were used to measure achievement. For this group of students, analyses indicated no significant overall effects of treatment on science achievement. A statistically significant effect was found between concept mapping and student achievement among the average students, as measured by combined performance assessment items. The results suggest that the effect of concept mapping on science achievement is not clear, but that lower ability students appear to have better success with concept mapping than higher ability students. (Contains 4 tables.)