Concern about viral contamination of groundwater by on-site wastewater treatment systems led to consideration of coliphages as indicators (Johnson and Cliver 1986; Snowdon and Cliver 1989) . In field studies with poliovirus introduced to groundwater via septic tanks, indicator bacteria (total coliforms, fecal coliforms, fecal streptococci) were not consistently present in groundwater samples in which the virus occurred (Alhajjar et al. 1988; Stramer 1984) . Since human viruses cannot multiply in the environment, it was important to show that candidate indicator coliphages could not multiply in the environment, either. We focused on FRNA (''male-specific'') coliphages, which are similar in size to the small enteric viruses and were detected in about half of septic tanks from various areas of Wisconsin (Woody and Cliver 1994) . These were not found in groundwater samples, even if taken directly under the soil infiltration field. Phage Qb was used as a model: it was found that F-pilus synthesis by host cells did not occur at temperatures below 25°C-a temperature not encountered in Wisconsin groundwater nor in many Wisconsin surface waters (Woody and Cliver 1995) . Also, the host E. coli cells had to be actively growing (log phase) to support Qb replication, which is less likely in the environment than in permissive conditions in the laboratory. Other constraints that were identified included competition from insusceptible bacteria with the host cells, which needed to be present at at least 10 4 CFU/ml to support Qb replication, competition from other phages, lack of nutrients in environmental waters, etc. (Woody and Cliver 1997) . We did describe a convenient viradel method for detecting waterborne coliphages (Jothikumar and Cliver 1997) , and a fluorescent plaque assay for coliphages from environmental samples .