The synthesis and properties of some new 7-cyclopropyl-4,7-dihydro-4-oxothieno[2,3-b]pyridine-5-carboxylic acids and the 7-tert-butyl analogues, incorporating substituents at positions 2 and 3, are described. The antibacterial activity of these derivatives and of the parent 7-alkyl-2-chloro compounds, unsubstituted at position 3, has been evaluated against an assortment of micro-organisms. 2-Chloro-7-cyclopropyl-4,7-dihydro-4-oxothieno[2,3-b]pyridine-5-carboxylic acid (3a) was found to possess leadingin vitroactivity among 4-thieno[2,3-b]pridinones known to date. However, none of the 3-nitro- or 3-amino-2-(4-methyl-1-piperazinyl) derivatives (4–6) showed interesting antibacterial activity.