Nanoparticles (<I 00nm) are being used in different applications such as biomedical, personal care, industrial, and environmental remediation. NZVI particles have unique physico-chemical properties like high surface area and high reactivity. High reactivity of nanoparticles is a concern when it comes to various ecosystem components. The impacts of NZVI particles on ecosystem components including endemic microbial community are not well studied. This research is an effort to elucidate the interactions between NZVI and some environmentally significant gram negative bacteria. Experiments were designed to study the interactions of three model gram negative bacterial species (Escherichia coli 8739, E. coli JM I 09, and Pseudomonas putida FI). The growth and viability of all bacteria were evaluated in the presence of NZVI particles with nutrients (in nutrient media) and without nutrients (in buffer solution). The bacterial species were exposed to various NZVI concentrations (0.09-10 mg/mL) under stirring (800 rpm) and shaking (150 rpm) conditions at different temperatures (4, 22, and 37°C). Microorganisms exposed to 0.09, 0.2, 0.5, 0.8 and 1.0 mg/mL of NZVI in buffer (stirring condition), showed variable viability. At very low NZVI concentration (0.09 mg/mL) viable cells were seen until 60 min, and at higher concentration (0.2-1.0 mg/mL) no viable cells were observed after 5 min. NZVI had no significant effect on the three strains at lower concentrations (] and 2 mg/mL) in the nutrient media. E. coli JM I 09 and P. putida FI were significantly reduced at environmentally significant NZV I concentrations. However, following an initial reduction E. coli 8739 recovered back to the same level as the control. The recovery of E. coli 8739 was not due to a reduction in the toxicity of NZVI as redosing with NZVI at 3 and 6 h did not affect the regrowth. Hence it was postulated that the observed regrowth in the presence of NZVI was a result of growth characteristics of the microorganism. Further experiment at suboptimal growth temperatures of 22°C and 4°C resulted in reduction in bacterial viability for both 5 and IO mg/ml of NZVI. Based on the results obtained from experiments done at suboptimal temperature and comparing them with the results from experiments conducted at optimal temperature of 37°C it can be concluded that NZVI toxicity on microorganisms are microbial species/strain specific and in some cases depends on the growth of the microorganisms. Actively growing E. coli 8739 are not affected by NZVI toxicity while non-dividing cells are adversely affected.