METHODS. Cultures. The rough strain of Sh. shigae and the CCY medium (Gladstone and Fildes, 1940) mentioned in an earlier paper (van Heyningen and Gladstone, 1953a) were used. The culture vessels were tubes shaped like an inverted T. This is a modification of an excellentdevice originally designed by Dr. Jacques Monod of the Pasteur Institute, to whom we are indebted (see Monod, Cohen-Bazire and Cohn, 1951). The horizontal limb of the T is a tube of 2-25 cm. external diameter and 16 cm. long, one end being rounded and the other invaginated by means of a sealed-in tube of 1-2 cm. external diameter and 4 cm. long. The invagination is used in connection with growth measurements described below. The vertical limb of the T is a tube of 1.75 cm. external diameter, 9* 5 cm. long, and open at the end. The tube is charged with5 to 12 ml. of medium, plugged in the vertical limb, and autoclaved and inoculated. It is then clamped on a rocking device with the centre of rotation 1 cm. below the plugged end, and continuously rocked in the plane of the tube through an angle of 350 at the