This paper reports the effect of inoculations with single and mixed Septoria tritici isolates on grain yield and 1000 grain weight of susceptible and resistant wheat genotypes. Nine bread wheat (Triticum aestivum L.) genotypes of different origin and showing different genetic responses to S. tritici Rob. ex. Desm. were inoculated under field conditions with three single isolates (P8, P9, and B1) and mixtures of those isolates. Reductions in yield and thousand kernel weight (TKW) were significantly higher in susceptible genotypes than in resistant ones. Single isolate B1 caused the highest yield losses in both susceptible and resistant wheats. The mixture P9×P8 caused significantly higher losses than single P9 or P8 inoculations in susceptible and resistant wheats, while isolate mixture P9×B1 caused significantly lower losses compared to P9 and B1 in both susceptible and resistant genotypes. Isolate P8 caused the highest losses in TKW of susceptible genotypes, while for resistant ones the highest losses were caused by B1 followed by P8. Isolate mixture P8×B1 caused the highest TKW losses in both susceptible and resistant genotypes, while P9×B1 caused fewer losses in resistant genotypes, but not in susceptible ones. The isolate mixtures significantly reduced losses in susceptible genotypes.