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Growth and photosynthesis of axenic and colonized Myriophyllum were compared to test the validity of using axenic plants as controls in the quantification of extracellular organic carbon (EOC) release. Axenic plants were characterized by lower growth rates that could be attributed to the unavailability of some major nutrients other than N, P, or C and (or) micronutrients in the culture medium. Vmax, the maximum rate of bicarbonate uptake, and Pmax, the maximum light-saturated rate of photosynthesis, of nonaxenic Myriophyllum were significantly higher than those of axenic plants. These differences could be attributed to epiphytic algal photosynthesis. At subsaturating dissolved inorganic carbon concentrations (below 15 mg C ∙ L−1), both plants achieved similar rates of photosynthesis but differed in the kinetics of EOC release. In short-term incubation (2–6 h), 14C-EOC accounted for 0.2–0.4% of photosynthesis, and total EOC amounted to 1.3–3.8%. 14C-EOC consisted primarily (≥ 60%) of low molecular weight products (≤ 1500). No differences were apparent in size distribution patterns of 14C-EOC from axenic and nonaxenic Myriophyllum and at different dissolved inorganic carbon concentrations. Axenic plants generally showed lower rates of EOC release (in absolute values). On colonized Myriophyllum, the contribution of the epiphytes to the EOC release pool was found to be low (≤ 20% of 14C-EOC) and could partly explain the greater EOC release rates of nonaxenic plants. However, our results are not totally conclusive because the lower growth rate of axenic plants could also be responsible for the lower photosynthetic and EOC release rates of these plants.