In analyzing populations of non-infected potato tuber moth (PTM) Phthorimaea operculella, using a total DNA probe from Phthorimaea operculella granulovirus (PhopGV), false positive reactions were obtained indicating homology between cellular and viral DNAs. Using a cloned 2.1 kbp fragment of PhopGV DNA, a specific digoxigenin-labeled probe was developed. This fragment did not show homology using both dot and Southern blot hybridization with either the genome of the larvae or genomes of the cell lines derived from the insect. The PhopGV-specific DNA probe detected as little as 1 ng, while the total DNA probe could detect even 35 pg of purified viral DNA. The 2.1 kb probe was highly specific for PhopGV. It gave negative results with two other granuloviruses isolated on Sesamia cretica and Spodoptera littoralis. The availability of a PhopGV-specific probe is an important prerequisite of detection of early stages of virus infection both in vivo on P. operculella larvae and in vitro on established P. operculella cell lines.