Abstract
Let M-g be the moduli space of smooth algebraic curves of genus g over C. In this paper, we prove that the set S-r subset of M-3 of moduli points of smooth plane quartic curves (nonhyperelliptic curves of genus 3) having at least one sextactic point of sextact multiplicity r, where r is an element of {1, 2, 3}, is an irreducible, closed and rational subvariety of codimensional r - 1 of M-3 - H-3 (where H-3 subset of M-3 is the hyperelliptic locus).