Water can be a vector of viral disease, but direct virological analysis of water has logistic and practical limitations. Viruses of major importance for water hygiene (e.g. hepatitis and gastro‐enteritis viruses) cannot yet be grown in tissue culture. Therefore, as in bacteriological quality procedures, model organisms are required for the evaluation of virologicalquality of water and the effectiveness of virus removal by water treatment processes. On the basis of published information, the F specific RNA (FRNA) phages have been chosen for this purpose. For the enumeration of the phages a particular Salmonella typhimurium strain with an artificially introduced F plasmid was developed as a host strain and was found to giveaccurate and reliable results. FRNA phages were found in very high numbers (102–105 pfu/ml) in all types of waste water investigated. FRNA phages are seldom found in non‐faecally contaminated waste water. Surprisingly low numbers are found in faeces.