The aim of this work was to investigate the role of a non pathogenic adenovirus (EDS-76, strain A-127), frequently isolated from ducklings on the infection by a pathogen agent (such as the parvovirus responsible for Derzsy's disease, DDV). Three groups of ducklings were respectively infected with A-127 alone, at day old, DDV alone at day 10th, A-127 at day old plus DDV at day 10th. The infected groups and a control one were raised under similar conditions. Pathological signs were observed and body weights recorded. Sera were collected weekly and analysed for DDV and A-127 antibodies. In addition, in vitro assays of the multiplication of these viruses and studies on cytopathic effects on duck embryo fibroblasts were conducted. The humoral response did not appear to be influenced by dual infection. No differences in cytopathogenic effects were detected in vitro between single and dual infection. On the other hand, in ducklings, dual infection increased the mortality rate and depressed the body weight gain, as compared to infection with either virus alone. The drastic effect on the body weight gain of the EDS-76 virus in the presence of DDV clearly shows a synergic role of this virus on the course of Derzsy's disease.