Mastitis is an inflammation of the mammary glands in the dairy market, caused by a variety of microorganisms such as fungi, bacteria and viruses. Microalgae are known as rich sources of biologically active metabolites, such as antibacterials. The objective of this work was to evaluate the antimicrobial activity of extracts of Chlorella vulgaris and Dunaliella tertiolecta against four bacteria, Staphylococcus aureus, Pseudomonas aeruginosa, Corynebacterium sp. and Escherichia coli using the microdilution method. Cell extracts were obtained by sonication with sterile water and sodium acetate buffer (SAB). In the results with C. vulgaris only the SAB presented MIC against the bacterial isolates, inhibiting the growth of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. In contrast, extracts of D. tertiolecta obtained with both solvents, water e SAB, presented similar results. The present study promotes the antimicrobial potentials of Chlorella vulgaris and Dunaliella tertiolecta against pathogenic bacteria isolated from mastitis.