A series of N-(1-methyl-1 Hpyrazole-4-carbonyl)-thiourea derivatives were assessed for their in vitro antimicrobial and anti-pathogenic activity against twenty-two strains of Erwinia amylovora isolated from different regions in Romania. The compounds were solubilised in dimethylsulfoxide and screened for their in vitro antimicrobial activity. The qualitative screening of the susceptibility spectra of various strains to the compounds was performed by adapted diffusion techniques (distribution of the tested compound solution directly on the solid medium previously seeded with the bacterial inoculums). The quantitative assay of the minimal inhibitory concentration (MIC, microg/mL) was based on liquid medium two-fold microdilutions. The subinhibitory concentrations of the tested substances were investigated for their influence on biofilm development on inert substrata. The present study showed that six new thiourea compounds exhibited a low antibacterial activity (MIC values > 500 microg/ml), but the subinhibitory concentrations inhibited the biofilm development on inert substrata. Thus, these results could suggest the usefulness of the tested compounds as control agents for preventing the first stage (colonization) of the infection with the fire blight pathogen.