- We explored the effects of learning two different symbolic sets of numerals (Arabic and Indian) on the development of automatic number processing. Children in the school we examined learn Indian numerals between first and third grades. In third grade, they switch to a new set of numerals (i.e., Arabic numbers). Participants in this study performed a numerical Stroop-like task in which they assessed the numerical value or physical size of stimuli varying along these two dimensions. Each participant saw either Arabic or Indian numerals. The results of the size congruity effect in the physical task, for both Indian and Arabic numerals, suggest that studying two sets of numerals interferes with the acquisition of an automatic association of a numerical symbol and magnitude. This is true both for the first learned set of numerals (i.e., Indian numerals) and for the second one (i.e., Arabic numerals). Furthermore, we found an absence of the distance effect, which further supports this conclusion. This learning program gave us the unique opportunity to examine the connection between symbolic sets and the mental representation of numbers in a novel fashion.