The viability of probiotic Lactobacillus rhamnosus HN001 was examined in fermented milk with and without Saccharomyces cerevisiae var. bayanus EC‐1118. The yeast enhanced the viability of L. rhamnosus at 30C but not at 12C. High temperature (40C) was detrimental to yeast vitality and diminished its viability‐enhancing effect. The yeast inoculated at 103 cfu/g multiplied to 106 cfu/g and enhanced the viability of L. rhamnosus. Viable yeast exerted maximum beneficial effects, although the cell‐free yeast supernatant also enhanced the survival of L. rhamnosus. The addition of yeast after fermentation decreased its viability‐enhancing effect. Removal of oxygen achieved the viability of L. rhamnosus that was comparable with that achieved in the presence of viable yeast. The results suggest that the improved viability of L. rhamnosus might involve yeast viability, oxygen removal and yeast metabolite(s). The findings would help develop novel microbial starter technology for ambient‐stable fermented milks with live probiotics. Practical Applications Maintaining cell viability of probiotics is a scientific challenge, especially under ambient conditions. The findings of this study would lay the foundation for developing novel microbial starter technology for ambient‐stable fermented foods with high cell counts of live probiotics, which entails stringent process and engineering control to ensure hygienic conditions and product safety. Benefits of such products would include: reducing distribution and storage costs by saving capital cost involved in chilled‐chain distribution, storage and retailing; market penetration and growth where it was not possible due to a lack of or poor refrigeration; extending shelf life and increasing consumer convenience.