The methods used for concentrating animal viruses from drinking water were found to be unsuitable for the concentration of bacteriophages from natural waters. The factors affecting recovery were investigated and a concentration procedure devised which is amenable to larger scale and field use. This procedure involves: (1) passage of the water through a sand filter; (2) removal of dissolved organic material with an anion exchange resin; (3) addition of MgCl2to a final concentration of 5 times 10‐4m; (4) adjustment of the pH value to 3°8; (5) adsorption of the bacteriophages on to fibre glass and cellulose nitrate filters; (6) elution of bound phage with 3% (w/v) beef extract, and (7) concentration by ultrafiltration of the resulting eluates. Using this procedure a wide range of test bacteriophages was concentrated from 41 to 5 ml with recoveries ranging from 18–80%—concentration factors of 200–