Fecal samples from 145 litters and 34 sows in 11 commercial pig farms in four provinces in Luzon were screened for the presence of Isospora suis. I. suis oocysts were identified , morphologically after sporulation of oocysts in potassium dichromate solution and counted by the modified McMaster technique. Positive litters were found in 40 (27.6%), 7-27 day-old litters in 10 (90.9%) farms. A prevalence of 13.6% was found in 5-7 day-old litters, 43.1% in 8-14 day old litters, 2t6% in 15-21 day old litters and 7.1 % in 22-30 day old litters. Fecal oocyst counts in infected litters ranged from 100-203,400 oocysts per gram of feces (OPG), with the highest counts recorded in 8-14 day-old age group (geometric mean 6.37 OPG) but differences between age groups were not statistically significant. Intensity of I. suis coccidiosis seemed to decline with age. However, age could only explain 1.65% of the variation in fecal oocyst counts. Diarrhea was recorded in 64.3% (18/28) of nursing pigs infected with I. suis, but was not found to be statistically associated with the infection. The results of the study indicate that I. suis was common in the farms surveyed and that sows did not appear to be a source of infection for these infected nursing piglets.