Experiments were done to examine the sensitivity of various waterborne bacteria from iodinated systems to iodine, and their subsequent recovery and growth, because this halogen is used as a disinfectant in potable water systems on US manned space vehicles. A Pseudomonas aeruginosa isolated from a commercial iodine product was least sensitive when grown in reagent-grade water or phosphate buffered water (PBW) and most sensitive when cultivated on mineral salts medium supplemented with low levels of glucose and glutamate or Brain Heart Infusion (BHI) broth. However, a P. cepacia strain was most sensitive when grown on BHI broth. Isolates from an iodinated potable water system were less sensitive to iodine than Ps-4 while a clinical isolate exhibited intermediate sensitivity. Bacteria including Ps-4 generally recovered and grew in PBW at greater rates than uniodinated controls. Following repeated iodination, PBW-grown pseudomonads became progressively less susceptible to iodine disinfection. Long-term cultivation in PBW led to a progressive loss of suscept-ibility to nalidixic acid. The sensitivity of pseudomonads to iodine was also examined following biofilm formation on stainless steel coupons. These experiments revealed that intact biofilms and cells removed from the coupons had significantly higher resistance to iodine than planktonic cultures. The disinfection efficiency of both iodine and chlorine were also studied using the waterborne respiratory pathogen, Legionella pneumophila. This bacterium was much more resistant when grown under low nutrient conditions and attached to stainless steel surfaces. Therefore, the susceptibility of waterborne bacteria to antibacterial agents including iodine and nalidixic acid is highly variable and dependant upon the organism, antecedent growth conditions and biofilm formation. Disinfection of low nutrient water may lead to the establishment of resistant bacteria within treated systems. Therefore, the design of disinfection studies should take into account antecedent growth conditions, ambient selective pressures, nutrient limitation and biofilm formation.