Fungistatic activity of a Lactic-Acid Bacteria (LAB) was assessed in vitro while isolated from its rumen fluid, against Fusarium oxysporum and Fusarium fujikoroi, basal-rot related species in yellow pitahaya. Fed batch was used to reproduce said LAB. Fungistatic activity kinetic tests were performed on the ferment (S1) and on two of the fractions obtained through centrifugation, supernatant (S2) and biomass (S3). The fungistatic activity was measured in two substrates, agar Man Rogosa Sharpe (MRS) and Potato Dextrose Agar (PDA). On S1 it was also performed lactic acid and acetic acid production kinetic tests as well as substrate consumption and biomass formation. Structural changes were evaluated on the fungi through microscopic electronics. The LAB was molecularly identified as Weissella cibaria. It was found that after eight hours of fermentation, fractions S1 and S3 performed the highest fungistatic activity against F. oxysporum and F. fujikoroi. Porosities were observed in the cell walls of the fungi as an effect of the LAB's fungistatic activity. These results indicate that LABs, when isolated from the animal habitat, may have a biotechnological potential to be used in the development of biocides, useful to control Fusarium in crops; for instance, to control yellow pitahaya basal rot.