The invasive mollusk Physa acuta is a usual host for the oligochaete Chaetogaster limnaei. This invertebrate has ectocomensal and parasitic forms, and its effects on the molluscan hosts can be both positive and negative. The ectocomensal forms of C. limnaei can feed on the intramolluscan larval stages of trematode digeneans, preventing new infections and controlling parasitic diseases. Taking into account the potential importance of the relationship between P. acuta and C. limnaei, and the role of the oligochaete for the control of digeneans, the objective of this study was to quantify the presence of ectocomensal and parasitic forms of C. limnaei in P. acuta, analyzing their levels of infestation in Los Padres shallow lake (Buenos Aires, Argentina). The samplings were carried out seasonally during spring and summer, and 2400 specimens of P. acuta were examined. During the study, only the ectocomensal forms of the oligochaete were found around the region of the head and foot of the snail. The intensity of infection was similar to the previous records in other parts of the world and, the highest prevalence values of C. limnaei during the spring, were consistent with the higher densities of P. acuta in this shallow lake, decreasing its values towards summer. The obtained results let assume a potential regulatory capacity of C. limnaei on the larval digenean assemblage of P. acuta.