We investigated the parasite community (arthropods and filarioids) associated with wild rodents from a marshy area in Buenos Aires province, Argentina. During 1995 and 1996, 105 sigmodontine rodents were trapped in The Reserva Selva Marginal de Punta Lara and in La Balandra (Argentina). Six sigmodontine species with a relative density (RDI) of 11.1% were trapped. A total of 3,371 specimens from 16 species of arthropods and 2 species of filarioids were collected. The values of parasite species richness (S) and prevalence (P) were as follows: from Scapteromys aquaticus S = 9; P = 96%, from Oligoryzomys spp. S = 8; P = 100%, from Oxymycterus rufus S = 8; P = 93%, and from Akodon azarae S = 6; P = 93%. The single Deltamys kempi captured was not parasitized. Parasite communities of S. aquaticus and Ox. rufus showed the highest similarity resulting from their overlapped microhabitats, not only when presence/absence of species was considered (Sorenson's index; Css = 82%), but also when the proportion of every species was taken into account (Pianka's index; O = 26.67). The 2 species of filarioids collected were restricted to Oligoryzomys spp. and Ox. rufus. The absence of lice and filarioids from Ox. rufus from La Balandra suggests an isolation of this host population from the northern localities. Considering arthropod–filarioid associations, a significant affinity was observed from Ox. rufus from Punta Lara, between Litomosoides oxymycteri and the mite Ornithonyssus bacoti. Scapteromys aquaticus and Ak. azarae may not be susceptible to filarioids; although Or. bacoti were recovered from both.