This study investigates the relationship of socio-economic status to the acquisition of two basic ideas underlying the concept of number conservation, "conservation of identity" and "conservation of equivalence." Tests for conservation of identity and of equivalence, detailed in the paper, were given to kindergarten, first, and second grade children from low-income and middle-class backgrounds. The study found a greater proportion of the middle-class children conserving equivalence, with significant differences at the first and second grade levels. On the conservation of identity test, the two socio-economic groups differed significantly at the kindergarten level. Results also indicated that the identity tasks were significantly easier than the equivalence tasks for children in both socio-economic groups. (Author/DT)