The prevalence of Isospora suis and clinical signs of isosporosis were observed in 33 swine farms from 20 sites in the southeastern state of São Paulo, Brazil. The study was performed by collecting 177 faecal samples from nursing and weaned piglets. A history of clinical neonatal isosporosis, as well as the type of farrowing and nursery houses and the pig management in the farms were correlated to the prevalence of I. suis oocysts. Six faecal samples were collected in each of the farms (two from groups of 10- to 19-day-old piglets, two from groups of 20- to 29-day-old and another two from groups of 30- to 50-day-old pigs). Faecal consistency was also registered at the time of their collection. Chi-square tests were used for statistical analysis. Oocysts were more prevalent in farms with a history of neonatal isosporosis than in those without previous cases. Faecal consistency was not related to oocyst elimination. In farms with a history of clinical isosporosis, faecal samples from groups of 10- to 19-day-old piglets showed a higher prevalence of oocysts than the groups of other ages studied. There was no difference in the prevalence of oocysts between nursing and weaned piglets. Oocysts were more prevalent in faecal samples collected from dirty-cemented floors than from self-cleaning floors in the farrowing houses. Types of floor and pig management in nursery houses were not associated with the presence of oocysts in weaned pigs.67169-74