Cavia intermedia is a rare cavy, endemic to the 10-ha Moleques do Sul Island, southern Brazil. We conducted the 1st study of reproductive and growth patterns for this species in its natural environment. Based on 17 months of monthly population monitoring using capture–mark–recapture methods, females were reproductively active throughout the year. Litter size was small (1 or 2 young per litter), with well-developed offspring that weighed approximately 19% of the mass of an adult female. Sexual maturity was reached later than in other species of this genus, at around 59 days of age or 70% of adult body size. There was no evidence that longevity was longer than for congeneric species. Other than longevity, these characteristics are consistent with the island syndrome, which may contribute to the persistence of this species.