Aspects of the mineral nutrition and growth of Myriophyllumn aquaticum were studied in the laboratory and field. Laboratory experiments were conducted to determine the importance of mass flow of sediment interstitial water, induced by evapotranspiration, in the supply of mineral nutrients M. aquaticum. Only about 2% of the water lost by emergent stems of M. aquaticum originated in the sediment; the remainder was taken up through adventitious roots in the water column. I concluded that mass flow of sediment interstitial water did not enhance nutrient supply to sediment roots. Water-use efficiency of M. aquaticum was similar to C-4 terrestrial plants. Criteria for assessment of N and P deficiency in emergent M. aquaticum were established in laboratory experiments and used to screen for deficiencies in naturalized populations. Growth of populations from high-nutrient drainage ditches was not limited by N or P by any of the criteria. Both N and P limited growth of M. aquaticum in a lentic population that was sampled over a three year period. N was more limiting early in the growing season and P more limiting later in the growing season. Productivity and tissue N and P concentrations were higher in shallow, near-shore areas than in deeper, off-shore areas. The gradient in productivity was attributed to the inability of adventitous roots, formed on rhizomes in the water column, to reach sediments in deep water. P marginally limited growth of M. aquaticum in a second lentic population sampled. Allocation of N, P and biomass in a naturalized population of M. aquaticum was examined over one growing season. Biomass allocation was primarily to rhizome tissues; emergent tissues never comprised more than 24% of the total biomass. Submersed stems with leaves were important components of the biomass only during winter. There was no accumulation of biomass over the growing season, suggesting that biomass loss rates equalled production rates. Seasonal changes in concentrations indicated that rhizomes stored N but not P and carbohydrates during the growing season. The allocation of P and the lack of P stores may make M. aquaticum succeptible to a harvesting strategy that removes only the emergent biomass, especially in P-limited stands.