This study describes a method of strength improvement of cement–sand mortar by the microbiologically induced mineral precipitation. A thermophilic anaerobic microorganism is incorporated at different cell concentrations with the mixing water. The study showed that a 25% increase in 28 day compressive strength of cement mortar was achieved with the addition of about 10 5 cell/ml of mixing water. The strength improvement is due to growth of filler material within the pores of the cement–sand matrix as shown by the scanning electron microscopy. The modification in pore size distribution and total pore volume of cement–sand mortar due to such growth is also noted. E. coli microorganisms were also used in the cement mortar for comparison, but no improvement in strength was observed.