The structure and absolute configuration of a unique alkaloid isolated from an Ecuadoran frog, Dendrobates tricolor, have been elucidated by X-ray crystallography and found to be 8-hydroxy-8-methyl-6-(2'-methylhexylidene)-1-azabicyclo[ 4.3.0]nonane. This indolizidine alkaloid (C16H29NO) represents the first structurally defined member of the pumiliotoxin A class of dendrobatid alkaloids. Analysis of mass spectra and proton and carbon-13 magnetic resonance spectra allows the formulation of structures of six further members of this class of alkaloids, including pumiliotoxin A (C19H33NO2) and pumiliotoxin B (C19H33NO3). An alio series of alkaloids in the pumiliotoxin A class is proposed to contain a 7-hydroxy substituent. The pumiliotoxin A class represents the fifth class of pharmacologically active and structurally unique alkaloids to be isolated from defensive skin secretions of Neotropical poison frogs (Dendrobatidae). © 1980, American Chemical Society. All rights reserved.