The responses of Escherichia coli to fosfomycin and fosmidomycin were investigated by continuous turbidimetric monitoring of cultures exposed to the drugs and by microscopy. The activity of both agents was potentiated by glucose-6-phosphate, suggesting that they share the inducible hexose phosphate transport system in Escherichia coli, but several differences of response were also detected: the inoculum effect was much smaller with fosfomycin than with fosmidomycin; inhibition of bacterial growth occurred much more rapidly with fosfomycin than with fosmidomycin; and fosfomycin was able to induce the formation of spheroplasts much more rapidly than fosmidomycin. Stable resistance to fosfomycin and fosmidomycin was readily induced in cultures of Escherichia coli, and some resistant variants retained susceptibility (or partial suceptibility) to the other compound. These observations suggest that although fosfomycin and fosmidomycin may be transported into Escherichia coli by a similar mechanism, the intracellular target site may be different.