Isotopic compositions of beogenic iron minerals may be used to infer invironmental conditions under which bacterial iron reduction occurs. The major goal of this study is to examine temperature-dependent isotope fractionations associated with biogenic siderite (FeCO3). Experiments were performed by using both mesophilic (<35°C) and thermophilic (>45°C) iron-reducing bacteria. In addition, control experiments were performed to examine fractionations under nonbiologic conditions. Temperature-dependent oxygen isotope fractionation occurred between biogenic siderite and water from which the mineral was precipitated. Samples in thermophilic cultures (45-75°C) gave the best linear correlation, which can be described as 103 Inαsid-wt = 2.56 × 106 T-2 (K) + 1.69. This empirical equation agrees with that derived from inorganically precipitated siderite by Carothers et al. (1988) and may be used to approximate equilibrium fractionation. Carbon isotope fractionation between biogenic siderite and CO2, based on limited data, also varied with temperature and was consistent with the inorganically precipitated siderite of Carothers et al. (1988). These results indicate that temperature is a controlling factor for isotopic variations in biogenic minerals examined in this study. The temperature-dependent fractionations under laboratory conditions, however, could be complicated by other factors including incubation time and concentration of bicarbonate. Early precipitated siderite at 120-mM initial bicarbonate tended to be enriched in 18O. Siderite formed at <30 mM of bicarbonate tended to be depleted in 18O. Other variables, such as isotopic compositions of water, types of bacterial species, or bacterial growth rates, had little effect on the fractionation. In addition, siderite formed in abiotic controls had similar oxygen isotopic compositions as those of biogenic siderite at the same temperature, suggesting that microbial fractionations cannot be distinguished from abiotic fractionations under conditions examined here.