IT is often advantageous for the microbiologist to know the pH limits within which a micro-organism will grow, and the influence of pH on the antimicrobial action of various drugs. Customarily, such studies are carried out in a series of separate vessels, each adjusted to a different pH-level. This note describes a modification of the Szybalski gradient plate technique1 so as to provide a pH gradient across the surface of an agar plate, thus facilitating studies involving pH effects on the growth of micro-organisms.