Reaction of olefins with CHFBr2 and BuLi gave stereospecific cis and trans isomers of alkyl-substituted fluorocyclopropanes (I, R, R1, R2, R3 = H, Me, Pr, C5H11). The addn. was also stereoselective with preferred formation of the syn isomers of I. Therefore, London dispersion forces seem not to be the origin of the syn stereoselectivity obsd. with carbene addn. reactions. [on SciFinder (R)]