This study was designed to investigate the cognitive level of development and mathematical fluency of first grade children. Piaget's conservation-of-liquid task was administered to 97 6- and 7-year-old children from two low-socioeconomic-level elementary schools. Using a counterbalanced design, conserving and non-conserving children completed separately timed lists of addition and subtraction problems. Two one-way analyses of covariance (ANCOVAs) were conducted to evaluate the fixed-factor of conservation level (conserving and non-conserving) and the two dependent variables (addition and subtraction fluency); age was a covariate. After controlling for the effect of age, the results suggest that conserving children were significantly more fluent in completing addition and subtraction problems than non-conserving children. Overall, children in the same grade and of the same age were at different levels of cognitive development; these levels had an effect on both addition and subtraction fluencies.