A species of sea hare was collected from the Mediterranean Sea, Alexandria, Egypt. It was identified based on general morphological and anatomical features as Aplysia fasciata. The antibacterial and antifungal activities were investigated via the standard techniques. Data obtained revealed that the highest antibacterial activity was detected against P. aeruginosa (AU = 3.4), followed by E. coli (AU = 2.9), then by B. subtlis (AU = 2.7). The other bacterial pathogens were not affected at all. Likewise, the maximum fungal suppression, via the pouring method, was observed against P. crustosum (50%). AUs against both F. solani and A. niger were 20 and 10%, respectively, while there was no activity recorded against the others. Also, the antifungal activity via the well-cut diffusion method conducted that the highest AU (6.8) was recorded against A. flavus, followed by AU = 4.8 against F. solani, then 1.8 against P. crustosum. Moreover, the antifungal AU against reference yeast strains ranged between 3.1 and 6.8. The highest one was recorded against C. tropicalis, followed by AU (4.8) against R. mucilaginosa. Regarding investigating the efficacy of some commercial antibiotics (mm), data confirmed that the Gram-negative bacteria were more resistant than Gram-positive bacteria. On the other side, the result of GC-MS/MS of crude extract observed the presence of several bioactive constituents, most of which had antimicrobial activities.