Incubating cultures ofEscherichia coliwith propionic acid (5 mmol/l) or formic acid (10 mmol/l) at pH 5.0 produced bacteriostasis lasting 30 and 120 min respectively. During this time rates of RNA, DNA, protein, lipid and cell wall synthesis were reduced. Growth resumed after continued incubation in the presence of acid, but cells from acid‐treated cultures were larger than controls. DNA synthesis was particularly sensitive to the presence of the propionic or formic aci