This study was conducted to identify an indicator organism(s) in evaluating the pathogen-reducing capacity of biogas plants. Fresh cow manure containing 10(4) to 10(5) colony forming unit (CFU) per milliliter of Escherichia coli and Enterococcus faecalis along with an inoculated Clostridium perfringens strain were exposed to 37A degrees C for 15 days, 55A degrees C for 48 h, and 70A degrees C for 24 h. C. perfringens was the most heat-resistant organism followed by E. faecalis, while E. coli was the most heat-sensitive organism. E. coli was reduced below detection limit at all temperatures with log(10) reductions of 4.94 (10 s), 4.37 (40 min), and 2.6 (5 days) at 70A degrees C, 55A degrees C, and 37A degrees C, respectively. Maximum log(10) reductions for E. faecalis were 1.77 at 70A degrees C (1 day), 1.7 at 55A degrees C (2 days) and 3.13 at 37A degrees C (15 days). For C. perfringens, maximum log(10) reduction at 37A degrees C was 1.35 log(10) units (15 days) compared to less than 1 unit at 55 and 70A degrees C. Modeling results showed that E. faecalis and C. perfringens had higher amount of heat-resistant fraction than E. coli. Thus, E. faecalis and C. perfringens can be used as indicator organisms to evaluate pathogen-reducing capacity in biogas plants at high temperatures of 55A degrees C and 70A degrees C while at 37A degrees C E. coli could also be included as indicator organism.