Positive interactions between species may change in outcome over time and may differ for the two interacting species. We explored the seasonal patterns of facilitation between an ascidian (Styela clava) and a macroalga (Undaria pinnatifida) by following their association for a complete sporophytic phase. In addition, we investigated how the relationship affected the morphometric parameters of U. pinnatifida, and whether the association had a negative outcome for S. clava. We performed monthly surveys in San Antonio Bay (Argentina) for nearly 1 yr to evaluate U. pinnatifida density (both the total and mature individuals exclusively) on different substrates to determine whether U. pinnatifida was more closely associated with S. clava than with inert substrates (i.e. consolidated and nonconsolidated). Moreover, we compared the morphometry of U. pinnatifida between substrates (S. clava vs. rock) and collected S. clava individuals overgrown by U. pinnatifida and classified them by their holdfast coverage to assess their condition (dead or alive). A higher total density of U. pinnatifida was associated with S. clava than with other substrates from autumn to spring. U. pinnatifida individuals growing on S. clava were smaller than those growing on rock. Finally, the probability of S. clava dying increased for individuals with higher holdfast coverage, although the scarcity of dead individuals found suggested a negligible influence. Our findings indicate that whereas U. pinnatifida clearly benefitted from this interaction by appearing earlier in the field and presenting higher densities that could benefit the population when associated with S. clava, for S. clava the relationship appeared more neutral.