Iron-oxidizing autotrophs (mainlyThiobacillus ferrooxidans) and acidophilic heterotrophs were recovered and quantified at an incubation temperature of 18 °C, in four tailings-effluent samples obtained from the environment of a uranium mine in Ontario, Canada. The samples were collected during winter when the temperatures of the effluents were in the range 0.5–5.0 °C. Iron-oxidizing autotrophs were recovered in the four samples and their numbers ranged from 3 ± 2 to 185 ± 18 colony-forming units/mL; acidophilic heterotrophs were recovered in three of the four samples and their numbers ranged from 13 ± 3 to 2517 ± 859 colony-forming units/mL. Forty-six of the iron-oxidizer isolates and 63 of the acidophilic heterotrophic isolates were examined for their ability to grow at temperatures of 4, 18, 21, and 37 °C. None of the isolates was psychrophilic, although 96% of the iron oxidizers and 54% of the acidophilic heterotrophs were psychrotrophs; less that 5% of the isolates for both nutritional types were capable of growth at 37 °C. In addition, the isolates were categorized as 'broader temperature range psychrotrophs,' 'narrower temperature range psychrotrophs,' 'intermediates,' or mesophiles, and the narrower temperature range psychrotrophs were found to be numerically predominant.Key words: psychrotrophs, psychrophiles,Thiobacillus ferrooxidans, acidophilic heterotrophic bacteria.