Ascidiella aspersa (Muller, 1776) is an exotic tunicate species widely distributed along the Sud-Occidental Atlantic coasts. In Patagonia waters, it forms dense aggregations at El Sótano (San Matías Gulf). We report an undescribed association between this invasive tunicate and a native mytilid species, Musculus viator (d'Orbigny, 1842). During a benthic study, a total of 543 individuals of A. aspersa were collected at El Sótano. A total of 689 individuals of M. viator were extracted from the tunics of 42 A. aspersa specimens. A maximum of 50 mytilid per tunicate was found. No relationship was found between the size of the tunicates and the size of M. viator. The prevalence of M. viator on A. aspersa individuals suggested that the tunicate invasion might have altered the community structure by improving local physical conditions -alternative substrate or refuge- for mytilid populations.