Number sense development of 411 middle- and low-income kindergartners (mean age 5-8 years) was examined over 4 time points while controlling for gender, age, and reading skill. Although low-income children performed significantly worse than middle-income children at the end of kindergarten on all tasks, both groups progressed at about the same rate. An exception was story problems, on which the low-income group achieved at a slower rate; both income groups made comparable progress when the same problems were presented nonverbally with visual referents. Holding other predictors constant, there were small but reliable gender effects favoring boys on over all number sense performance as well as on nonverbal calculation. Using growth mixture modeling, 3 classes of growth trajectories in number sense emerged.