Based on the morphologic characteristics of oocysts of the genus Isosporn, we have demonstrated that I. ohioensis is a relatively common Isosporn species in dogs which are resident in Chonbuk province, Korea. The prepatent period of 1. ohioensis was four days. The size of the unsporulated oocysts in fresh stool specimens was $22.9{;} imes{;}19.8{;}mu extrm{m}$ (R = 1.16). The size of the contracted sporonts was $17.4{;} imes{;}16.3{;}mu extrm{m}$ (R = 1.061). By 96 hours, sporulation is complete and the ratio of length/width was constant relatively. And the sizes of oocysts and sporocysts were $22.8{;} imes{;}20.5{;}mu extrm{m}$ (R = 1.11) and $15.0{;} imes{;}10.8{;}mu extrm{m}$ (R = 1.39), respectively. At this time it is most reliable for the measurements of oocyst sizes of 1. ohioensis to provide with the identifiable clues against the others. It is therefore recommended that the clinical fecal specimens suspected of isosporosis should first be incubated and aerated for 96 hours before a definitive parasitological diagnosis can be reached. These and other observations contribute to the understanding of the biological characteristics and laboratory and clinical diagnosis of isosporosis.