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For any simple graph G=(V,E) with n vertices and a k-labeling f:E→{1,2,...,k}, the vertex sum w(v) on a given vertex v is the sum of labels of all edges adjacent to v. f is called a modular irregular k-labeling if the vertex sums on G are all distinct resudue modulo n. The smallest positive integer k for which G admits a modular irregular k-labeling is called the modular irregularity strength of G. In this paper we determine the modular irregularity strength of prism graphs.