Specimens of Ixodes longiscutatus Boero, 1944 (Ixodidae) were collected in Uruguay and, to a lesser extent, Argentina from 1998 to 2004. Specimen data and literature records were used to compile a comprehensive list of hosts and to define the ecological parameters of this tick species, which is currently known from a total of only 6 females, 37 nymphs and 94 larvae. The male remains undescribed. Bona fide records of females were from cattle, horse, Cavia tschudii Fitzinger, 1857 and Cavia sp. (Rodentia: Caviidae). Most nymphs were collected on rodents of the family Cricetidae, especially Oxymycterus nasutus (Waterhouse, 1837) and Scapteromys tumidus (Waterhouse, 1837), but also C. tschudii. Again, most larvae were found on cricetid rodents, namely O. nasutus, S. tumidus, Akodon lutescens Allen, 1901, Oligoryzomys flavescens (Waterhouse, 1837) and Necromys obscurus (Waterhouse, 1837), and on Caviidae (Cavia aperea Erxleben, 1777 and C. tschudii). Both larvae and nymphs were found on hosts during all seasons of the year, with a higher prevalence in summer. Two allopatric populations of I. longiscutatus appear to exist, one in Uruguay and neighboring provinces of Argentina (Buenos Aires and Entre Ríos), the other in northwestern Argentina (Salta Province); DNA analysis may confirm their conspecificity. Scanning electron photomicrographs of the capituli of larvae and nymphs, as well as the posterior setae of the nymphal idiosoma, are provided.