ABSTRACTCyprinid herpesvirus 3 (CyHV-3), a lethal DNA virus that spreads in natural lakes and rivers, infects common carp and koi. We established a quantification method for CyHV-3 that includes a viral concentration method and quantitative PCR combined with an external standard virus. Viral concentration methods were compared using the cation-coated filter and ultrafiltration methods. The recovery of virus-like particles was similar for the two methods (cation-coated filter method, 44% ± 19%,n= 3; ultrafiltration method, 50% ± 3%,n= 3); however, the former method was faster and more suitable for routine determinations. The recovery of seeded CyHV-3 based on the cation-coated filter method varied by more than 3 orders of magnitude among the water samples. The recovery yield of CyHV-3 was significantly correlated with that of the seeded λ phage, and the average ratio of λ to the CyHV-3 recovery yield was 1.4, indicating that λ is useful as an external standard virus for determining the recovery yield of CyHV-3. Therefore, to quantify CyHV-3 in environmental water, a known amount of λ was added as an external standard virus to each water sample. Using this method, CyHV-3 DNA was detected in 6 of the 10 (60%) types of environmental water tested; the highest concentration of CyHV-3 DNA was 2 × 105copies liter−1. The lowest recovery limit of CyHV-3 DNA was 60 copies liter−1. This method is practical for monitoring CyHV-3 abundance in environmental water.