Pyrrolizidine alkaloids (PAs) are produced by numerous plant species in nature. Numerous intoxications in animals and humans caused by the consumption of certain plants were attributed from the middle of this century to compounds of vegetable origin, the pyrrolizidine alkaloids. Phytochemical studies on the crude extract of the leaves of Crotalaria species (Fabaceae family) were conducted. These plants are commonly used in traditional medicine such as for treatments of uterine hemorrhages, dysentery, and inflamed wounds. Although it has medicinal value, it was also reported that leaves of this plant containing pyrrolizidine alkaloids (PAs) are toxic to the humans and animals, which are hepatotoxic, pneumotoxic, genotoxic, neurotoxic, and cytotoxic. The purpose of current investigation is to study the PAs content of Crotalaria species. The structure of desired phyto constituents were elucidated using Liquid Chromatography-Mass Spectroscopy and a combination of one- and two dimensional 1H and 13C Nuclear Magnetic Resonance spectroscopy. In this study, one potent pyrrolizidine alkaloids, monocrotaline was isolated from Crotalaria retusa. This results suggests that it should be a safety consideration in consuming this plant for traditional medicine because it is also contains toxic pyrrolizidine alkaloids. The structurally elucidated toxic compound, monocrotaline was used for further toxicological investigations.