Novel 2-chloroethanephosphonic acid and 2-chloroethanethionophosphonici acid derivatives of the formula ##EQU1## wherein X is oxygen 2-chloroethanethionophosphonic sulfur; and R is aryloxy, optionally substituted with hydroxy, alkyl, halogen, or nitro, or monoarylamino, or monoalkylamino of from 1 to 6 carbon atoms; exhibit marked plant growth influencing properties, e.g. inhibition or stimulation or alteration of plant growth.