A review of the structural identification and bioactivity of simple indolizidine and quinolizidine alkaloids isolated from amphibians, ants,fungi, plants and marine sources, covering the period from 1994 to 1999,is herein presented. Some of the new alkaloids do not have complete structural assignments due to the minute amounts that were isolated and the proposed structures rely on GC-MS and GC-FTIR analyses. Some previously known structures were corrected and others now have complete spectral assignments. The configurational and conformational analysis of new alkaloids is discussed and the alkaloids whose structures were established by X-ray analysis are also presented. Indolizidine and quinolizidine alkaloids from amphibians and ants have been described as noxious compounds and are believed to play an important role in the self defence system of these animals. Their biological activity is likely to arise from interference with ion channels in nerve and muscles cells. Polyhydroxylated indolizidines form an important group of bioactive alkaloids. Different structures are referred as potent inhibitors of glycosidases, as immunosuppressants, as anticancer agents and others are described as anti-HIV agents. Lupine alkaloids (quinolizidine alkaloids from Lupinus species) are associated with plant ecological equilibrium because they are important protectors of plants against herbivores, microorganisms and competing species. They are responsible for the toxic and teratogenic effects observed in livestock and theiraction can be associated with their affinity for nicotinic and muscarinic receptors. Important bioactivity of specific alkaloids is outlined.