Escherichia coli can cause health problems in humans and livestock. It is frequently found in faeces and soil, both of which can contaminate grass harvested for silage-making. To determine the survival of E. coli 0157:H7 during ensilage, and its influence on ensilage, unwilted precision-chop grass was ensiled in laboratory silos with the following additive treatments: (a) no additive, (b) E. coli 0157:H7 (Ec - non-infectious strain, at log10 4.5 colony forming units (cfu) g-1), formic acid (FA), and (d) Ec + FA. Silos were stored at 15oC. Three silos per treatment were opened on days 0, 2, 5, 9, 19 and 180 of ensilage. Silages made without additive or with formic acid underwent contrasting but rapid, lactic acid dominant fermentations. Formic acid restricted fermentation, reducing (P\u3c 0.001) buffering capacity and the concentration of lactic acid and increasing (P\u3c 0.001) the concentration of water soluble carbohydrates. Counts of indigenous Enterobacteria were initially high (log10 8.1 cfu g-1) but declined rapidly in the early stages of ensilage and were not detected by day 19 of ensilage. Formic acid increased (P\u3c 0.05) the initial rate of decline in enterobacterial numbers. No indigenous E. coli 0157 were found on the ensiled grass. Inoculation with E. coli 0157:H7 increased (P\u3c 0.001) the numbers of this organism but they declined rapidly and were absent by day 5 of ensilage. The addition of formic acid accelerated (P\u3c 0.001) this rate of decline. The added E. coli did not alter (P\u3e 0.05) silage fermentation pattern