Several oomycete species are pathogenic to plants or animals, including humans, and cause economic and environmental damage, nevertheless no effective molecules against these pathogens are available. Dimethomorph (DMM), is a fungicide with high activity against plant pathogenic oomycetes used to control downy mildew. The current study aims at developing a transversal approach and to do that, we evaluated: 1) the effect of two concentrations of DMM on in vitro mycelial growth on Saprolegnia and Pythium isolated from fish and aquatic environment; 2) the effect of DMM on hyphal morphology of S. parasitica and on the sporangia and hyphae of a P. viticola strain using Scanning Electron Microscopy. Results indicated that radial growth of Saprolegnia species was reduced at 50 and 100 mg/L of DMM after 24h of incubation. For P. viticola at 50 mg/L sporangia are rarely present with respect to the control and in 100 mg/L are not present at all. Moreover, a SEM protocol was developed, and allowed to observe how the fungicide induced changes at the morphological level in both oomycete species.