Cyanide could not be detected in the rhizosphere of potato with the methods used. However. the assimi- lation and detoxification of cyanide by soil micro- organisms (Gastric, 1981) makes the chances of demonstrating microbial cyanide in the rhizosphere by our technique very small. This does not necessarily detract from any supposed effect of microbial cyanide on root cell metabolism. Much of the cyanide produced by bacteria in direct contact with root cells, will not be available for microbial degradation. as it most likely diffuses directly into the root cells. Cyanide may also be produced by pseudomonads in the endorhizosphere, where they can be abundant