In the sea anemone Bartholomea annulata, four different types of cnidocysts, basitrichous isorhizas, microbasic p-mastigophores, microbasic amastigophores and spirocysts were identified. In relation to the efficacy of different substances to induce discharge of nematocysts we observe that distilled water induced more than 70% of microbasic p-mastigophores to discharge, whereas spirocysts were discharged in a lesser extent (≈20%). The median lethal dose (LD50) in mice was found after injection of 700.7 mg protein per kg of body weight from the crude extract. The protein with neurotoxic effect was isolated using low-pressure liquid chromatography. The neurotoxic activity was determined using sea crabs (Ocypode quadrata), injecting 15 μg of crude extract or isolated fraction into the third walking leg, and violent motor activity followed by progressive loss of sensibility to external stimuli, further leading to full paralysis were observed. The active fraction (called V) eluted at 43.9 min.