An attempt was made to develop an ecosystems approach for the control of the nuisance aquatic plant Eurasian watermilfoil, Myriophyllum spicatum L. To this end, a cellulolytic fungus, Mycoleptodiscus terrestris (Gerdemann) Ostazeski (M.t.), and a pectinolytic bacterium, Bacillus sp. strain P8 (BSP8), were isolated from the microbial populations naturally resident in the phyllosphere of this plant. These organisms grew compatibly with each other, were able to compete successfully with other microflora on the plant surface, and were able to resist the inhibitory action of phenolic compounds produced by the plants. Application of these organisms to the plant, after growth in appropriate media, resulted in plant decline and eventual death. The process of decline included hormone-like stress effects on the plant induced by BSP8 (internodal elongation), and increase in strongly pectinolytic microbial populations associated with the plant, and penetration into plant tissue by the fungal mycelium. Specificity trials of M. t. infectivity revealed it to be very weakly pathogenic to several aquatic species and terrestrial plants, and thereby without potential significant impact outside the area of designated application.