Based upon 1 H-NMR, MS and IR spectra, a perhydro 2-allyl-5-ethyl-7-npropylpyrrolo[2,1,5-cd]indolizine structure (1) is proposed for a major tricyclic alkaloid 261C isolated from skin extracts of a Madagascan poison frog of the mantellid genus Mantella. Brightly colored poisonous frogs of the mantellid genus Mantella are found only in Madagascar. We published a survey in 1993 1 of alkaloids in seven species and this included many of the pumiliotoxin and homopumiliotoxin classes, many bicyclics of the "izidine" class and several decahydroquinolines. Most have been reported in New World dendrobatid frogs although others, as yet, have been found only in the Madagascan frogs. One quinolizidine alkaloid, 217A, was isolated in sufficient amounts for a complete structure determination by 1 H-NMR spectrometry. 2 We also published in 1996 3 a tabulation of alkaloids present in skin extracts from various populations of nine Mantella species but characterization details of a number of previously unreported alkaloids were not presented. Recent gas chromatographic-EI-MS spectral studies of skin extracts from a population of the Madagascan frog Mantella betsileo indicated the presence of significant amounts of a tricyclic alkaloid of molecular weight 261 with the following EIMS spectrum: m/z 261 (19%) with major fragments at 260 (27), 232 (77), 220 (64), 218 (100), i.e. ions consistent with losses of hydrogen, ethyl, allyl and propyl radicals from the molecular ion. The code designation 261C had been assigned to this alkaloid, when it was first detected as a major alkaloid in skin extracts of another mantellid frog, Mantella expectata. At that time it was designated as having an unclassified structure. 3 Both species containing 261C were from near the Massif Isahlo Reserve in an arid region of southwestern Madagascar. An HRMS spectrum indicated a molecular formula C18H31 and MS measurements of the above fragment ions (see Fig. 1. GC-FTIR spectrum of 261C (1)) were consistent with the proposed fragmentations (see EXPERIMENTAL). A CIMS spectrum with ammonia confirmed 261 as the molecular ion mass and a CIMS spectrum with ND3 indicated no exchangeable hydrogens. A gas chromatography-Fourier transform infrared (GC-FTIR) spectrum (Figure 1) indicated the presence of a terminal olefin (913, 992, 1639, 3083 cm-1) and only a very weak Bohlmann band was observed at 2811 cm-1. The molecular formula indicated four rings or unsaturations. One double bond was indicated by hydrogenation. A 2.8:1 mixture of two compounds of molecular weight 263 was detected by GC-MS spectral analysis after hydrogenation. Both compounds showed only fragmentations for loss of ethyl and propyl groups (EXPERIMENTAL). Consequently 261C 1 has a tricyclic structure with ethyl, propyl and allyl substituents. A minor dihydro compound of mol. wt. 263 was present in the original extract where it had cochromatographed with 261C. It is given a code designation of 263G. Approximately 0.9 mg of 261C was purified from pooled skin extracts of a population of M. betsileo 4 using flash chromatography and HPLC. Various 1 H NMR experiments provided data shown in Table 1 and discussed below. Unfortunately the sample was contaminated by some 28 % of the naturally occurring dihydro congener assigned code designation 263G, which could not be separated from 261C under all HPLC conditions tried. Alkaloid 263G must have a different stereochemistry than 261C since it separated on GC-MS analysis from dihydro-261C. This contamination, as well as the overlapping of many signals, even at 800 MHz, made analysis difficult and the proposed structure (1) has some inherent ambiguities and must be regarded as provisional. An impurity triplet is seen at δ 1.09 overlapping the triplet assigned to the methyl of an n-propyl group. In addition, the multiplets at δ 2.27 and 1.89-1.92 contain impurity peaks. Any attempt to collect more of these frogs from their original habitat proved impossible, as spraying to eradicate locusts with insecticides had killed the frogs and presumably most of the insects upon which they were feeding. Stomach contents of certain species of Mantella frogs of Madagascar have been recently studied and are found to be comprised chiefly of small ants 5 . However, as yet no tricyclic alkaloids, similar to 261C, have been described from Madagascan ants.