Screening extracts of the Mexican cereoid cactus Backebergia militaris by mass-analyzed ion kinetic energy spectrometry (MIKES) initially indicated the presence of alkaloids with (M+H)+ 194 and 208. Subsequent analysis using analytical thin layer chromatography (tlc) detected an additional secondary amine (3) in the alkaloid extracts, which was resolved from previously isolated alkaloids (1 and 2) via column adsorption chromatography and identified as lemaireocereine (7,8-dimethoxy-1,2,3,4-tetrahydroisoquinoline), an aromatic substitution isomer of heliamine (2). Additionally, 3-methoxytyramine (4) was crystallized as the hydrochloride from the phenolic alkaloid fraction. Cyclizations of primary β-phenethylamines (1 and 4) at positions 2 and 6 are likely involved in the biogenesis of the isomeric tetrahydroisoquinolines lemaireocereine and heliamine. Trace alkaloids with (M+H)+ 208 were not isolated due to insufficient plant material but are presumed to be a mixture of the 2-methyl derivatives of heliamine (2) and lemaireocereine (3).