The eco-friendly synthesis of silver nanoparticles (AgNPs) is reported from the reduction of an aqueous silver nitrate solution (10 mM) using a natural extract of guava leaves (Psidium Guajava). The properties of the synthesized nanoparticles were determined by SEM and UV-vis spectroscopy, showing an average particle size of 31±7 nm, with the presence of the wavelength peak at about 414 nm. Antimicrobial capacity was evaluated using Escherichia coli ATCC 25922 through the diffusion disc method, showing inhibition zones up to 1.5 mm with concentration of 3.1 mg/mL of nanoparticles. The toxicity of AgNPs was evaluated using the nematode Caenorhabditis elegans as biological model exposing at different AgNPs concentration (500, 1000, 2000 and 4000 µg/L). After 24 h of contact time no significant toxicity effects were observed on mortality, locomotion, and growth, suggesting the potential of these nanoparticles for biological applications.