The phenolic compounds of microalgae act to inhibit the growth of toxigenic fungi, this study had as objective to prove the action against the production of trichothecenes, a group of mycotoxins produced by Fusarium, that normally contaminate cereals. The microalgae phenolic extracts (Spirulina sp. and Nannochloropsis sp.) were extracted with methanol and resuspended in water. The in vitro experiment consisted in adding the phenolic extract of each microalga (40 μg mL-1) in petri dishes containing dextrose potato agar and previously sterilized wheat grains, with subsequent inoculation of a strain CML 3607 mycelial disc. The control was cultured with sterile water. A treatment with the fungicide tebuconazole (0.6 mg mL-1) was also performed. Petri dishes were incubated at 25°C, light/dark photoperiod of 12-12 h. After 168 h the samples were extracted by the adapted QuEChERS method; the identification and quantification of trichothecenes (nivalenol, deoxynivalenol and acetylates) was carried out through high performance liquid chromatography coupled to the ultraviolet detector. The antimycotoxigenic effect of the extracts application showed mainly against nivalenol, when the phenolic extracts of the microalgae were applied, the characteristic peak of the mycotoxin was not detected (suggesting total inhibition). When used tebuconazole there was an increase in nivalenol content of approximately 15%. Both microalgae phenolic extracts also had a promising effect on the inhibition of deoxynivalenol, reaching no detection (with Nannochloropsis sp. extract) and 62% reduction (with Spirulina sp. extract). The application of the fungicide tebuconazole resulted in an increase in the deoxynivalenol concentration. Both microalgae phenolic extracts and tebuconazole inhibited the production of acetylates, but again the natural extracts showed high levels of inhibition, reaching 78% for Spirulina sp., while 34% was the maximum inhibition with Tebuconazole. In this way, phenolic extracts of microalgae showed antifungal and antimycotoxigenic potential in cultures in vitro, while tebuconazole was less efficient.