Two stereoisomers of benzo[a]furo[2,3-g]quinolizine derivatives were obtained by alkylation of stereoisomeric 7a-methyl-2-oxo-5-phenylfuro[2,3-c]piperidines with methallyl chloride and subsequent cyclization of alkylated products in the mixture of trifluoroacetic and concentrated sulphuric acids. It is interesting to find that the cis-quinolizine, upon heating to 150o C in glacial acetic acid, is isomerized to the trans-isomer.