The apparatus which I have devised for the counting of bacterial colonies in roll-cultures, consists of a glass tube 15 centimetres long, the surface of which is divided by finely etched lines into square centimetres. The bore of this tube is such that the roll-culture tubes, which are of a uniform diameter, just slide without play within it. The ends of the tube are fitted into brass collars (Al, A2), which have inserted in them india-rubber rings (B1, B2). These india-rubber rings serve to retain the culture tubes in position during the process of counting. One end of the apparatus is adapted for grasping ordinary roll-culture tubes, the other for fixing the thin stems of Roux's tubes for anaerobic cultures.