The relative importance of spatial visualization ability and cognitive development for achievement in a geometry course for preservice elementary teachers was investigated. Both factors correlated significantly with achievement, but in an analysis of variance, only the main effect due to cognitive development was significant. There was no interaction between the factors. The effect of the semester-long geometry course on students' spatial ability was also investigated. It was found that the students' spatial visualization ability was significantly greater at the end of the geometry course than at the beginning.