Microalgae are becoming increasingly popular in various scientific fields due to their ability to produce a wide range of organic compounds, among which many have bioactive properties. Among the mentioned compounds, those with antimicrobial and anti-adherence activities are particularly interesting. Such microalgae also include Arthrospira platensis and Chlorella vulgaris, whose bioactive properties against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Candida albicans strains were researched in our study. We determined antimicrobial activity with minimum inhibitory concentration (MIC) of microalgae extracts with the broth microdilution method. We also determined antiadhesive activity with crystal violet staining method, for which we used sub-inhibitory concentration (0.1 MIC) of microalgae extracts, as we did not want to influence the growth but only adhesion of selected microorganisms. The results of antimicrobial activity showed that only CMAS and CMAC extracts (CMA: extraction with mixture of chloroform, methanol, acetone; S: A. platensis; C: C. vulgaris) were effective against S. aureus (MIC = 10 mg/ml), while other extracts had no antimicrobial effect against selected microorganisms (MIC > 20 mg/ml). In the continuation of our study, we found that MC extract had an antiadhesive effect against S. aureus (28.8% inhibition), while CMAS extract increased adhesion of S. aureus and MS extract increased adhesion of P. aeruginosa and S. aureus. This could be attributed to an error in absorbance measurement due to dark coloration of both extracts. Results of our research, together with results of researches of other authors proved that A. platensis and C. vulgaris microalgae represent a great potential in development of new and effective bioactive agents, but further researches are needed.