The water physico-chemistry, hydrology and plant communities in the Montgomeryshire Canal were examined for evidence of successional phenomena over a five year period and comparisons were made with previous studies. Plant communities changed in a manner which suggested that classical successional ideas were applicable to canals. The changes were readily distinguishable when a period of twenty years had passed but as the examined period was decreased the complexity of the sequence became more apparent. In a single year there were changes in the plant communities. A later stage of succession was indicated as the number of changes increased and the available energy and habitat niches were filled. Succession did not take place in a slow orderly manner but progressed as a series of jumps when the plants were released from inhibitory factors. These sudden changes were promoted by changes in the habitat due to climatic variation, removal of grazers or alterations in the water level. Where the most important inhibitory factors were unchanging, the succession proceeded to a climax community decided by those factors. The physico-chemical factors affecting the canal changed during the study period. The canal became more acidic due to the effects of a cold wet spring. The underwater light climate in un-navigated lengths was determined more by the amount of shade upon the water surface than by any other factor. Levels of suspended solids were relatively unimportant. Established plant communities showed a great resistance to change, despite being subjected to traumatic perturbations. Skim dredging of a site with a well established community structure did not change the course of the succession, but removed inhibiting factors such as anoxic material or grazing and promoted a more rapid change to reedswamp. In the .long term falling water levels had the same effect as raising of the bottom due to infill, but in the two routes towards a less aquatic system were different. Infill led to a moving band of depth sensitive plants whilst loss of water encouraged rapid creation of a reedswamp. The study confirmed that limited navigation is not detrimental to the species composition or the productivity. Eutrophication was a limiting factor creating an unique climax community.