In the present work the results on antibiotic activity of the extracts from the liverwortConocephalum conicumand the mossesMinium undulatumandLeptodictyum riparium, tested against 8 bacterial strains pathogenic to man, are referred to. The extracts have been obtained, according the method suggested by McClearyet al. (1960), in either organic solvents or in water solution. The extracts have been proved active on both bacteria Gram + ve (G +) and Gram—ve (G—), in addition the extracts fromConocephalum conicumandLeptodictyum riparium(extracts in organic solvents in particular) were the most effective in the tests. The acetone extracts fromLeptodictyum ripariumshowed the highest value of inhibition againstPseudomonas aeruginosa(bacterium responsible for serious opportunist infections). Our results on antibiotic activity of Bryophytes only partly agree with literature for either species showing activity or for the intensity of such activity. Among the three bryophytes employedLeptodictyum ripariumshowed the best inhibitory power against all bacteria tested. This result seems particularly interesting in relation to the possible competition between species occurring in the water where this moss preferentially grows. The phenomenon of antibiosis in the bryophytes is discussed also in relation to symbiotic or commensal relationships existing with soil bacteria and/or fungi.