When host bacterium H3 is cultivated in artificial seawater a t a concentration of 0.6 mg organic nutrients ml-l, its reaction to infection by phage H3/1 changes dramatically with the duration of incubation before phage attack. Cells infected when stlll In their logar~thmic growth phase rapidly produce progeny phage until breakdown of the phage-sensitive population. For cells infected after entering the stationary phase, rate and extent of phage propagation by resting cells decreases for some time, but both parameters rather suddenly increase again for cells infected after prolonged ~ncubation. Phage production may then reach the same level attained by cells infected during logarithmic growth phase; although, with increasing phage concentration, resting cells very effectively become protected from phage attack by pseudolysogeny. This course of events is mainly influenced by the method of incubation of the host (shaken vs still), by the age of the cells before ~nfection, and presumedly by changes in physiological traits of the bacteria when serially subcultured for extended periods of time. The reduct~on in volume of bacterial culture by repeated w~thdrawal of aliquots was found to be of minor importance; however, phaga production was measurably affected by the transfer of cells to fresh receptacles. With cells which entered thcir stationary phase up to 36 h before infection, a n initial phage concentration of between and 10 PFIJ (plaque forming units) ml-' was found to determine the extent, but not the rate, of phage production. The aforementioned observations were also made with cells stal-ved for 3, 8, and 22 additional days before inoculation, except with ~nitial PFU concentrations of and 10' PFU ml-l, when an appreciable increase in phage production was found. In cell suspensions seeded with phage 48 d after set up of the experiment, the highest phage production was found with the lowest initial phage concentration and vice versa. This finding, i.e. the inverse relationship between production and initial concentrat~on, is not in agreement with any of the current hypotheses concerning bacteriophage ecology. K E Y WORDS: Virus. Phage Bacteria. Reproduction