A comparative analysis of cellulase activity of 36 fungal strains of phytopathogenic and endophytic Ceratocystis sp. was conducted. The rate of their linear growth on the media with carboxymethylcellulose was studied. It was shown that the rate of linear growth of phytopathogenic strains on the media with carboxymethylcellulose was lower than that in endophytic ones. There was no correlation between the levels of cellulase activity of studied strains and rates of their linear growth. The majority of Ceratocystis sp. strains had middle but not high cellulase activity, cellulase activity varied in the group of endophytic strains more than in the phytopathogenic one. The differences in cellulase activity were observed on the strain level. No distinct dependence of cellulase activity on different growth terms of fungi was demonstrated. The distinct dependence of cellulase activity level of endophytes on the species and organs of host plant was not observed. The cellulase activity level of endophytic strains Ceratocystis sp. was lower than in endophytic Fusarium poae strains and similar to it in phytopathogenic Fusarium strains. Consequently the investigated endophytic Ceratocystis sp. strains can be classified as latent pathogens, which were able to cause the diseases of host plants in favorable for them conditions. The paper is presented in Ukrainian.