Psychotria forsteriana A. Gray is a species growing in Vanuatu (New Hebrides). The alkaloidal content of its leaves was extracted with alkaline ethanol, purified by organic solvents and chromatography on alumina, and finally separated through semi-preparative HPLC. Four optically active amorphous alkaloids (B, C, D, E) were isolated. For three alkaloids (B, C, E), physicochemical data, UV, IR and ¹³C-NMR spectra were identical with previously described compounds: Alkaloid B was similar to Quadrigemine B, Alkaloid C to Quadrigemine A, and Alkaloid E to Psychotridine. Elementary analysis, mass spectrum and ¹³C-NMR spectra suggested Alkaloid D is a new steric isomer of the isopsychotridine series, named Isopsychotridine C, which differs from Isopsychotridine A and B by UV, IR spectra and mass spectrum fragmentations. This preliminary study shows the alkaloids of P. forsteriana have the same basic skeleton as those previously isolated from other Melanesian species of the genus Psychotria. Spectral data support the assignment of Isopsychotridine C as a new stereoisomer of Isopsychotridine A and B, though the amount of isolated alkaloids was insufficient for complete stereochemical identification. Preliminary examinations of pharmacological activities of these alkaloids on tumor cells and on aggregation of human platelets were very concluant and will be exposed in further publications.