In Brazil mercury is used indiscriminately at prospecting sites for gold extraction, with a consequent enormous discharge of this metal into the aquatic ecosystem. Mercury concentration was investigated in 1995, 1996 and 1997 in the sediments of ten sites along the Carmo stream, MG, Brazil, almost all of them located in prospecting areas. Analysis of mercury in the sediments of all sampling sites showed that the levels were above the tolerable limit (0.1 mu g g(-)1), except for sites P4 (1997), P6 (1995) and P9 (1995 and 1997). The results showed wide mercury contamination in an environment not limited to the active or inactive prospecting areas, but also including sites outside these areas but downstream from them. There was a high incidence of bacteria resistant to mercury in the aquatic communities of the sites under study, ranging from 27.3 to 77.1%, except for P1 (an ecological station upstream from the sites under study) in which all bacteria isolated f rom water were sensitive. Furthermore, the fall in mercury concentration in the sediment at site P5 was not as marked as at other sites, with a frequency of resistant bacteria of only 27.3%, possibly indicating a slower detoxification. The statistical analysis (Pearson's correlation = - 0,527) showed that the hypothesis about negative correlation between the incidence of bacterial resistance and the total mercury concentration in this environment is supported. Hence, all these data denote a moderate association between the distribution of resistant bacteria and the presence of mercury compounds.
