An attempt was made to assess the hitherto undescribed optimal gaseous conditions for growth of zoonotic Helicobacter felis, focusing on the ratio of spiral-forms amongst the whole cells examined. The largest mean colony diameter was obtained under the gaseous condition of O(2) 12% and CO(2) 10%. In analyzing the five day old colonies, the highest percentage of spiral forms (85.5%) was observed under the condition of O(2) 18% and CO(2) 5%. In contrast, the lowest percentage of spiral forms (2.3%) was demonstrated under the condition of O(2) 1% and CO(2) 10%. The condition of O(2) 12% and CO(2) 10% was concluded to be optimal for obtaining cells with the largest colony sizes, although colonies proliferated under such conditions definitely contain many more coccoid cells than spiral forms. In culturing H. felis strains, optimal gaseous conditions should be employed according to the purposes or preferences of study designs.