This work describes the composition and distribution of sclerobionts on the Miocene Patagonian oyster Ostrea alvarezii d'Orbigny and provides information about its paleoenvironment. The structure of sclerobiont assemblages and their preferential distribution on left/right valves (n=340) as well as their external/internal surfaces and areas was evaluated by means of multivariate analyses (nMDS, SIMPER), non-parametric tests for producing mean comparisons, and goodness of fit tests using coverage and abundance data. Sixteen ichnotaxa were recorded. Bivalves, polychaetes and bryozoans were the most representative encrusters. External surfaces, left valves and sediment-free areas were the most colonized. O. alvarezii beds are interpreted as within-habitat time-averaged assemblages. Oysters appear to have lived in small subtidal channels in a low energy tidal flat and closely related to the sediment-water interface, with the right valve resting on the substrate. Sclerobionts showed diverse behaviors, trophic levels and tiering. This suggests that sclerobiont assemblages on O. alvarezii had a complex structure with subdivision of resources, including a vertical colonization of valves by tracemakers at different levels of depth.