As M. tuberculosis strains develop resistance to fluoroquinolones, pools of M. tuberculosis sensitive to drugs of this group and pools of M. tuberculosis with different resistance determinants can simultaneously coexist in the host organism. The goal of this research was to run an in vitro investigation of growth characteristics of M. tuberculosis strains which have different genetic determinants of resistance to fluoroquinolones, in the setting of competition for nutrients. The research used five clinical strains of multidrug-resistant M. tuberculosis differing in gyrA structure. Strains were cultured in pairs and individually under optimal conditions (Middlebrook 7H9 medium) and under conditions of multistress (50% Middlebrook 7H9 medium, 2 mM KNO₂, 0.02% H₂O₂). The experiment took 21 days. The number of cells of each co-cultured strain was estimated from calibration curves. These curves showed the dependence of the threshold cycle of the polymerase chain reaction — respective to the channel targeted by the mutation — on the concentration of M. tuberculosis cells. The competitive fitness value and specific growth rate were calculated from the number of cells of each strain when co-cultured. M. tuberculosis strains with mutations in gyrA were found to be inferior in growth rate to the wild-type gyrA strain, which was particularly pronounced under multistress conditions. The strain with the most common gyrA_D94G mutation had the lowest growth rate of all strains examined. It has been hypothesised that the slow growth of M. tuberculosis with this mutation may lead to tolerance to anti-tuberculosis drugs, and as a result, the strain gains an advantage under chemotherapy conditions compared to other gyrA mutant variants.