Zinc oxide nanoparticles were synthesized using acetic acid and ethylene glycol as solvents. Their characteristics were determined using IR and UV-Vis absorption spectroscopies, X-ray diffraction and transmission electron and scanning microscopies. Each solvent was found to produce ZnO nanoparticles with a different morphology and not monodispersed: the acetic acid solvent (with a heat treatment at 400 degrees C) gave acicular (with a size of similar to 40 nm x 500 rim) particles, while ethylene glycol (and a heat treatment of 600 degrees C) produced a spheroidal (similar to 40 nm) morphology. The antifungal action of the ZnO nanoparticles on Aspergillus niger and Botryns cinerea fungal strains was then determined. Those with acicular morphology showed a more efficient antifungal activity for the concentrations considered. High resolution optical microscopy images of imprints of the cultures studied showed that when these two fungi were subjected to the action of nanoparticles, alterations in the reproductive structures (conidiophores) and hyphae could clearly be seen.