1. Lysogenic B. megatherium 899a was adapted to growth in a minimal ammonium sulfate medium (ASCM).
2. Adaptation took place slowly and the following changes in the culture occurred:
(a) The growth rate increased from 0.5 to 1.5–2.0/hr.
(b) The culture changed from diffuse to mucoid.
(c) The total phage titer, and the gelatinase concentration decreased to 1/100 or less.
(d) The types of phage produced changed from >99 per cent T (wild type) to 30 to 60 per cent miscellaneous clear types. The original T phage was replaced by a different smaller t, never observed in the original 899a culture.
(e) Several new colony types also appeared, but the colony morphology was not correlated with the phage types produced. None of the colony types was stable on repeated transfer either in peptone or ASCM, but continued to disassociate into different colony types (cf. Ivánovics, 1955).
3. Control experiments showed that these changes in phage production and colony types could not be brought about by growing sensitive B. megatherium in the presence of the various new phages, in ASCM. It is therefore unlikely that the changes observed in adapted culture were due to infection of a sensitive cell with phage.
4. Continued growth of the ASCM-adapted strain in peptone resulted in increasing the total phage titer, and also the gelatinase concentration. The growth rate returned to its original value and the ability to grow rapidly in ASCM was soon lost. The phage types, however, remained the same as in the ASCM.
5. An improved cell for steady state growth is described.