A 1,8-naphthyridine compound of the formula ##STR1## wherein R is a vinyl or 2-fluoroethyl group, andR.sub.1 and R.sub.2 are the same or different and each represents a hydrogen atom or a lower alkyl group having 1 to 4 carbon atoms,and the esters thereof and salts thereof. For example, 7-(3-amino-1-pyrrolidinyl)-6-fluoro-1,4-dihydro-4-oxo-1-vinyl-1,8-naphthyridine-3-carboxylic acid and nontoxic salts thereof, which are covered by the aforesaid compound, are useful as an antibacterial agent.