A static, 14 day laboratory bioassay using axenic Myriophyllum sibiricum Komarov was developed and utilized to determine the effects of two reference toxicants [zinc chloride (ZnCl₂) and phenol] upon the morphological and physiological development of this species. For ZnCl₂ the highest “no observable effect concentration” (NOEC) was 4.26 mg/L for root length. Based upon IC₅₀ values, the most sensitive endpoint parameter for ZnCl₂ was root length (range = 8.2–17.8 mg/L), followed by fresh weight (range = 13.2 – 32.4 mg/L) and root number (range = 13.3 – 38.6 mg/L). For phenol, the NOEC was 11.76 mg/L for three endpoint parameters (root number, total root length and fresh weight). Based upon IC₅₀ values for phenol, the most sensitive endpoint parameter was root length (32.6 – 40.0 mg/L), followed by root number (45.8 – 57.0 mg/L) and fresh weight (48.2 – 75.9 mg/L). For both reference toxicants, the results were repeatable over time. As tested in this axenic culturing system, the ZnCl₂ was slightly more toxic to the M. sibiricum than the phenol.