Yogurt is manufactured with 1.25% of active Lactobacillus delbrueckii ssp. bulgaricus and 1.25% Streptococcus thermophilus in milk by weight. Yeast contamination can occur during and after manufacture of yogurt products through unsanitary ingredients, equipment and processing personnel. Lactobacillus paracasei (Lp) and Lactobacillus rhamnosus (Lr) have been known to possess antimicrobial and heat resistant activities, respectively, against the growth of yeasts in cultured dairy products. The objectives of this study were to determine effects of the addition of Lp and Lr on inhibition of yeast cell growth in yogurts, and evaluate the improvement of organoleptic quality and storage stability of the fermented products. Two separate experiments were conducted by additions of Lp and Lr strains at 3 different inoculation rates of 3 × 10¹, 3 × 10² and 3 × 10³ cfu yeast/2L yogurt and control group without bacterial addition. All control and experimental yogurt samples were stored at 2 temperature treatments (10°C and 25°C). The number of yeast cells in all experimental yogurt samples were counted weekly, and the swellings occurred due to gas production by contaminated yeast cells in all yogurt samples were evaluated. The results showed that the Lp and Lr bacteria added yogurt groups showed much slower rates of yeast cell growth and lower frequency of swelling in comparison with those of the control yogurts, suggesting that the growth of yeast was inhibited by the additions of Lp and Lr strains to the experimental yogurts. With respect to the effect of storage temperature, the yogurts stored at 10°C had little post-acidification in the Lp and Lr added groups compared with control samples, and also showed lower post-acidification than samples stored at 25°C. These outcomes suggest that the addition of Lp and Lr bacteria to the yogurt products could improve their organoleptic quality. It was concluded that the fortification of L. paracasei and L. rhamnosus bacteria improved storage stability and organoleptic properties of the yogurts, attributable to the delayed or inhibited yeast cell growth and post-acidification in the products.