Introduction of the ColV,I‐K94 plasmid into any of four strains ofEscherichia coligave derivatives which grew less well than the parent at alkaline pH. The alkali sensitivity of the 1829 derivative resulted from the presence of the plasmid rather than from its introduction into a less alkali tolerant variant. Of two other ColV plasmids tested, one (ColV‐K30) conferred substantial alkali sensitivity whereas the other (ColV‐41) had little effect. Of several other plasmids examined, R124‐F2 (which confers derepressed transfer properties) resembled ColV,I‐K94 in its effect on alkali sensitivity and ColV‐K98 produced a marked effect. The other plasmids (Flac, R124 itself, R1 and R483ColIa) had only a small effect. For ColV plasmids, it appears to be the presence of transfer and colicin components together which leads to the reduced tolerance to a